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The important comparison is to a profession with an equal amount of arms and essence, e.g. a warrior mage. A magician/fighter very likely will be better than a warrior mage with an equal number of total levels.
As of now, there is no such thing as a Warrior Mage in RMU.
But for the sake of the argument, let's use a Bard or Dabbler: At level 20, that character will have an OB of something like +110 from skill ranks alone (40 ranks at a cost of 3/5, or 8 per level).
Compared to a Fighter who switched to Magician at level 10 (or vice versa) and is now level 20: 12 points per level for 2 ranks in the primary weapon as a Magician, 4 points per level for 2 ranks as a fighter. Average cost: 8 Points per level! No difference there.
Now, if the latter concentrates for ten levels on one thing and for another ten levels on the other thing, he will of course have gotten more out of his DP, but at the cost of having lower total skill ranks in any given skill, which means to have less power (though more versatility). It's nice for some character concepts, but hardly game breaking. Seriously, a pure Magician at level 20 will "break the game" more.
Still, I wouldn't want to go as far as allowing to just spend DP to switch profession, as it is a way of thinking that is learned early on one's life that would have to be changed, along with many personal interests and thus maybe even a bit of the personality. It does, however, make total sense to have some Mentalism caster develop spells for such a thing as soon as people are aware that there are different "learning types" who get certain things faster than others.
But I really don't see how such a spell would be 50th level. ChatGPT (which I, btw, asked for explanations of Rolemaster Unified concepts before, and it answered largely correctly) suggested 7th or 8th,. That seems okay, considering the power level of other closed mentalism spell lists at that level, such as teleporting 100 meters. Maybe it's harder to manipulate the mind of a subject than to break the laws of physics entirely, so 10th level? 15th if we're being restrictive?