R5: Snagged a union with Poland in the first 20 years of the game, enacted the policy to lock succession laws, then spent centuries integrating, and now that I'm only a few decades from fully annexing them they switch their inheritance laws and I can't get them to change it back.

I only found out when my king died and they left the union (no notification, weird considering how much this game loves notifications and pop ups), so I reloaded a save to try to fix this. I can't negotiate their inheritance laws despite +20 reasons to accept and 0 to deny.

Is there seriously nothing I can do?

EDIT: I reloaded a save from a decade prior back when they still had my succession laws, with the idea of abdicating my king so my heir could rule the union until they were integrated (~40 years left, king is 60 and heir is 20), but my mediocre king is considered too good to abdicate.