Forcing a personal union on a republic in a war already changes them to a monarchy through the on_war_lost on_action since version 1.30. But there are only very few ways how you can even use the CB against a republic, because all missions which give a PU CB should use valid_for_personal_unions_trigger to make sure that the target is a monarchy (do you know any missions which are missing this check?) and even if you get the CB, you would lose it unless the target has the ambrosian_republic reform or the flag neapolitan_republic.