So its not possible to go from feudal to republic then? Shame.You can do that. He will not be your heir no matter what and anyone that becomes his heir will also not be able to inherite anything from a feudal ruler. Still its not recomended to do as it can create buggy situations. The mechanics are the same in any culture or religion with republics.
Is that not a (somewhat gamey) way of 'disposing' of a bad heir then? Gift him a Republic and skip to his genius brother?