Will change in diet cause change in activity?

As I've said in many other threads (I'm a little excited) I'm getting my brother's 12 year old tabby, Wick, tonight. I've been visiting her a lot to get her used to me, and I've noticed that she's really ... sluggish. Maybe just lazy. She's a bit overweight, and apparently pukes occasionally. This is, my brother says, the normal state for Wick. She has toys, mostly featherballs, and she'll chew on the feathers and sometimes bat once or twice if you try to get her to play, but she's just not playful. Now, I know, since she's 12, this could just be "I'm old, no kitten stuff for me" and if this is the case, it's just fine. I don't want to make her do things she doesn't want to do. But I'm changing her diet from crappy dry food, free fed, to two premium wet meals a day. Do you think this will change her energy levels at all? I'm fairly sure it'll stop the puking (she hates dry food anyway, but will eat it if there's nothing else), and she might even lose some weight, which would be good since she's getting up there in years. But will she play more? Have any of you had an experience with this sort of thing?

it certainly can change it! we had that happen with our little Phoebe when we got her. except it was the other way around. she was very malnurished and skinny and in horrible health. we got her up to speed and healthy and now she's just like a kitten!

Change her food slowly.. mix little bits of new in with the old and try diff. flavors. She may not react well to the new food and that will help with the transition. Cats often throw up if there's something 'allergic' in their food, sometimes just a little additive (who knows!) but may also just be sensitive, so changeovers should be slow.