Recorming should instantly also change the succession laws for the highest ranking titles of the ruler who did it.
Not for human players but for the AI. Humans always handle any law, The AI doesn't.
So germanic to feudal electiv or primo, slaves to oldst member of dynasty, tengri to youngest...
Just that bigger realms could come to existence.
I would personally prefer that the reformer gets to choose a better succession law rather than it defaulting to something based on religion/culture, especially as the religion/culture combo of the reformer could be extremely strange. I agree that the reformer should get a free switch from EG, at the very least, but beyond that I'm not sure how much of an improvement they should get (especially as the Christian world is likely to be stuck with Gavelkind/Elective at the time of a reasonably quick reformation).