Isn't it rather irksome when you nearly kill a lichen colony and then it merges with a tiny one nearby and recovers ALL of its HP?
I think it would be nice if damage and status carried over to monsters when they transform. This would make Lichen colony and Devilite fights much easier fun since you don't feel like you are wasting your time attacking an enemy when it will just eventually transform and heal itself completely.
I think when a transformation happens, the monster should retain 50% of the damage taken. If a lichen colony keeps having other little lichens combine with it, it'll heal but not to full immediately.
Retaining some damage is good, but otherwise you don't prioritize preventing the enemies from doing the transformations in the first place. Takes out the little bit of fighting strategy that those monsters have going for them(lichens especially).