The save-swap is insanely good, but restricted by being a 1/rest thing. But it's still insanely good.
If I were going to do this, I'd probably nerf it to "the first save changes type". So you can make Hold Person with *one* str save, or dex save, or whatever, increasing your initial chances, but it reverts for any future saves. This is a big deal for continuing-effect spells with saves every round.
I think that is a fair change. It is in line with the parts where it is clear certain spells were not considered (AOE + 2d10, etc)