"What kind of changes can I expect or are usual after achieving union with the Higher Genuis or Holy Guardian Angel (HGA)?"
Most people who attempt and succeed usually notice an increase in intuitive knowledge (or 'gnosis' I suppose you could call it). You still end up having to practice with your natural psychic abilities though. Just because you're psychically 'wired' or 'hooked-up' with your higher aspects doesn't automatically mean that you'll understand the non-verbal impulses or messages you'll be getting from them. Practice is still very necessary. In fact some people might not notice any changes at all at first simply because they haven't learned to speak the 'language' of their higher-aspects. If you're expecting intellectual types of thought (thought-with-form i.e. language or an audible voice) you might end up disappointed. Each person's experience is different, but from my own experience, there was no audible voice and it wasn't a separate entity. It was an aspect of me that I didn't know existed and that had previously only been felt in the background in the form of occasional sudden 'knowing' or intuition.
Prior to the conscious decision to achieve a 'union' I was granted a glimpse at my higher aspects (when I still believed the Higher-Self to be a separate entity) and the vision I was thrown into was of a silvery-white space while staring at a pearl brighter than any star. The feeling of that place was that there was a 'light' that penetrated my every thought evoking a feeling of intense devotion and harmony. I just wanted to scream out "Yes!!! Yes!!! This is what I long for!" Because I had been resting and was in a relaxed and receptive state, the higher aspects of my consciousness were able to grab hold of the lower/human aspects and just make a sudden 'connection' like that. I think this was done to give me a taste of what was possible and to make me hungry for more. Those spiritual aspects still wanted to make me work for the goal rather than just giving me what I wanted from what I could see. (Probably a test to make sure I was worthy of the extra responsibility).
"I have heard of possible personality changes, etc. But I know that this is an essential first step towards true enlightenment and will open doors for meaningful work in the higher magickal arts."
Personality changes can be an understatement! A lot of people start acting really screwy or become religious fanatics because they neglected the initial self-purification step of the process. By not being balanced (balanced in their energy/aura) and 'pure' (i.e. still retaining false views of reality and not resolving internal emotional/mental conflicts and past traumas) those people were not able to handle or correctly interpret the higher frequency energies and consciousness that was being channeled down through their human/personality aspects. If you look at past programs/methods of union, you'll probably find a similar theme in some of them. First, there is a balancing/purification step that gradually clears away the crud in the person's human personality (thereby creating a more perfect 'vessel' for the spirit) and then the person is gradually inoculated with higher energies so as to adapt to them in a balanced way. Then, the actual conscious union/channeling is attempted. There are many variations to this but I'm just trying to keep things simple in this post.
For the most part, a lot of people are already 'one' within themselves. What this work does is to cause the person to consciously become aware of that unity and of the source of their consciousness, while channeling energy/consciousness from higher aspects without it becoming distorted during transmission. The human personality/consciousness/ego is a type of 'projection/extension' into lower frequencies of reality BY/FROM the higher spiritual aspects. The mistake most of us make is forgetting our spiritual nature and focusing entirely on the physical environment to the exclusion of the 'spirit' from which we came. This is normal due to the physical demands of our environment and as a result a human ego is created to deal with them. This ego/personality creation (that part of you that might call itself 'Joe' or whatever your name is in the physical realm) usually makes the conscious spiritual union/remembering process rather difficult though. In order to channel the higher aspects you (as an ego) must relinquish your power to the spirit (i.e. your spiritual core/essence). The true higher 'you' won't always share the same ideas about what is necessary in life as the lower 'you' might. In some ways you could think of the channeling process as being similar to lining up a series of gears in a machine. You have to get the lower gears to line up and spin at the same rate as the higher ones in order to mesh with and be driven by them.
Let's say you have an old Pepsi bottle with stale flat soda in it. What you're going to do is to swish that stuff around in that bottle and then dump it out- icky floaties/chunks and all. Then you're going to rinse it out with fresh water and make it perfectly clean and receptive to a new substance. You'll also probably remove the Pepsi logo and maybe even do some sculpting to the inside and outside so that it's virtually a brand new bottle. THEN you're going to get in contact with a dealer of the world's finest wine (i.e. your 'Higher-Self' or the divine core/essence of your being). When the bottle/vessel is completely ready, you're going to gradually pour some of that expensive wine into the prepared bottle to see how well it can contain the new substance. (If the bottle cracks or falls over then you'll have to return to an earlier step in the process and go back to creating a suitable vessel all over again before re-attempting.) Then, you'll practice savoring the wine a little bit at a time while developing your sense of taste and smell so as to fully appreciate and understand the qualities of that wonderful vintage. (In other words, developing and practicing with your psychic skills and learning to understand the new spiritual 'knowledge'.) Eventually you'll be able to handle larger quantities of the wine without becoming 'drunk' (i.e. psychologically 'screwy').
Does this make better sense now? The concept is more simple than most books will try to make out, but actually DOING IT is sometimes very difficult because most people don't really WANT to make such a drastic change in themselves. To make yourself into a perfect vessel for your higher spiritual aspects, you have to actually let go of some of the useless emotional baggage that you might have been clinging to for many years. Some people don't want to change their thoughts and behavior because they fear what friends and family might think of them if they suddenly start acting and thinking in a completely different way.
"I am studying different methods and theories about this at present and I am giving careful thought before I go with one of them. I am really fascinated by the work, "The book of the sacred magic of Abramelin the mage". But it is an ardous path to take to achieve union with one's higher self with his 6 month method."
Nothing worth having is ever easy ;)
Actually, the Abramelin text is very difficult to follow and I know of only one person who ever tried it exactly according to the book's methods. I believe it was William Bloom that wrote a book called 'The Sacred Magician" and it's a diary of his results. You might want to pick it up if that subject interests you.
There are many ways in which to accomplish any one thing, and no one necessarily needs to spend the 6 months to accomplish that goal. I've found that such excessive time is necessary due to the methods of that text i.e. devotional prayer etc. If ceremonial magick is used instead, the process can take as little as a month or a few months depending on the person and his/her initial state of psychic 'connectedness' to those higher portions. My reasoning for this reduction of time is that ceremonial magick makes conscious directed use of energy to cause the change rather than waiting and wishing for the change to occur on its own. Both methods can be effective though and neither is perfect for everyone.
I achieved my initial state of 'union' a few years ago (no I'm not trying to brag and I have no intention of enhancing my ego here - just relaying information for others who might need it) through a combination of methods. I went through extensive self-purification with elemental energies and learned the basic theories of general magickal work. After I had practiced and studied formally under the supervision of a couple skilled practitioners I decided (and felt) that it was time to begin my serious work through union with the 'higher-self'. After some general devotional work and study on spiritual things to begin to align myself with my higher-will I then commenced a month-long series of intense daily invocations using a variation of the Bornless Ritual (see G:.D:.). To hasten the process, one of my mentors also worked directly on me to assist in the 'rewiring' process. (In his opinion, this is nothing more than an energy rewiring process - I tend to agree having seen and felt the results of his work.)
What did I feel and see??? At first nothing changed. I perceived everything the exact same way most humans do, other than an intense feeling of bliss in my heart chakra for a day or so. My views (misperceptions) of the process and how to use my consciousness at the time tended to hold back my development unnecessarily. Something I did notice right away though, was an increase in my ability to sense and work with energy in a psychic way. Whenever I practiced any psychic exercises, my sessions were far more effective and easy and I was able to 'touch' things astrally and actually feel them, whereas before it had always been a struggle. I also had an increase in guidance from the core/essence of my being and since I could now 'see' or 'know' the difference, I tended to do fewer things that were not in accordance with my true nature or 'true will'. This extra intuition has saved me from 'screwing myself over' in life countless times already. I also had a different view of other humans and no longer viewed them without compassion as mere objects to be used for my pleasure and advancement.
I'm reminded of the Hindu word for greeting and parting, Namaste, which means "that which is divine in me recognizes and acknowledges that which is divine in you." That concept comes close to the reason for my change in behavior.
"Does anyone have any thought on this topic?"
I think that if someone is at least 18 - 20 years of age (so as to ensure that they will not injure a psyche that has not finished fully developing) and they have the desire to go through with it, then union with their higher-aspects is certainly a noble cause.
Note: In retrospect, I would now place that age around 25 at least - 40 years of age or more is better.
In my opinion, it's the only sensible step to take toward learning magick and can eliminate years of unnecessary time wasted with inferior spellbooks and grimoires, but I admit to being a bit biased on that subject.