+10 merging question

So I have a +9 4* draug rn and I finally got the last one, but it's not the iv set that I ideally want. Can I merge a +10 character into another for their better iv set and you just won't get any more benefits from merging or will the game not allow the character to be merged any more? My +9 Draug is +atk -def, but I wanted a +spd draug (go figure, I wouldn't get a single +spd draug having summoned over 11 of them).