I don't doubt that Serge was a tennis coach. It was a long time ago so of course there are no websites with active business details, but traces remain if you look:
As far as how he 'transformed', according to an interview I read (which is no longer available on-line) SB claims he started hearing voices (from the Hierarchy) telling him who he had been in past lives and that he could be a great healer in this life.
On that note, it would be curious to know whether any of the UM students, particularly the long term ones, have started hearing voices as a result of connecting to the Hierarchy? It happened to Serge before he even began to change his diet and lifestyle habits so you'd think that after years of applying themselves to the 'Livingness' some of the students would be experiencing the same supernatural communications. If Serge is no different, greater or more special than any other person as his students always claim, why has nobody else made communication with the Hierarchy? Long-term followers of UM would do well to 'ponder on that'.
For a detailed account of Serge's transformation see here:
Note that he states he undertook healing courses and immersed himself in New Age literature:
"The ‘investigative’ nature or necessary part of the ordeal led him to having to read or look up books he would not normally choose in addition to choosing to participate in a series of ‘healing’ (so-called) courses. The books he had to read were not his cup of tea and, the awareness he was experiencing, very easily exposed the many hidden tricks, channelled delusions and plain old lies that presented themselves as truths in these ‘best-selling’ books. The unexplained knowing about subjects he did not know or cared about extended itself to those he sought expert understanding from and, whilst attending the courses he thought he had to attend, he likewise felt a deep dissatisfaction with their content, knowledge and integrity, something that to him made no sense at all since he was, after all, a novice learning the ropes, so to speak. Then, one day, at a course, he had an experience that was also witnessed by one other participant. The ‘experience’ was on one level profound, rather supernatural or phenomenal and yet, at the same time natural and, curiously, knowing, even if unusual for his healthy scepticism to contend with. This experience led to a deeper and more whole connection to and with his already developing deeply profound inner-feelings that were bringing the extraordinary level of awareness if not the re-awakening that was so astounding, but nonetheless weird at the same time."
The sceptical observer might suggest that this was done to familarise himself with the techniques and 'language' of the New Age milieu so that he could appropriate the elements that were to his liking (Alice A. Bailey's writings, for example), but also differentiate his own venture from the other, similar systems on the market.
An example of this would be the Hindu concept of Prana as adopted by many New Age teachings and modalities:
Serge inverts the information and deems Prana as bad, rendering all other systems teaching otherwise not only wrong, but harmful. Thus only the modalities offered by UM can truly help the individual seeking healing.