if you allow switching, there is no point anyone every chosing a semi as a profession - it's cheaper and more effective to chose a pure arms user, develop hits & weapons, then chose a pure spell user and develop the spells.
Any profession can learn any skill (and any spell)
The root of the issue of switching is that people associate profession with "what I'm currently doing" ... in that case, you start as a child, then student, then ...
I've yet to see "infant", "school child" or "student" as professions.
The way RM is designed - whether you agree or not - is that the "profession" describes how easy you can learn skills. It does NOT describe what you learn or what you do on a day to day basis.
Yes - if you take that to it's ultimate conclusion, everyone should be a "layman" - but that assumes that everyone learns in the same way. I don't believe that we do.
A warrior/fighter is the most "extreme" profession - they simply are essentially incapable of learning magic.
Thieves and Rogues can learn magic reasonably well ... semi spell users are better ... pure magic users are better again
Essence spell users really cannot learn weapons effectively (but at a cost of 9, their cost is 3 time less than a fighter can learn an individual spell).
Additionally - beyond obviously different professions, many professions differ in such minor amounts, and almost solely focused on differences in spell lists, that it makes a lot of sence to introduce more generic magic using professions that can then select (and/or change) base lists from a wider list than currently available - much like has been proposed for clerics/preists in the RMSS Channeling Companionm or for Mages/Magic users in one of the Rolemaster Classic expansions ...