Well, she'll still be able to fly and be fabulously gorgeous, but her levitation is going to be a strain. Sleep will only work for small periods. She won't be able to call up brambles/mega growth, but she can still talk to treeants/emote vibe at the trees. I'm going to say she can still transform Diaval, but that's a collaborative thing since there's basically a channel run in his soul/body/whatever that's used to switching back and forth (since you can see the difference between his first change and the ones he does near the end of the movie), so changing Diaval from raven to man and back will be the same, but changing him or anyone else into something other than what they originally are used to is going to be taxing and will leave her tired after.

I think that's a fair decrease.