I mostly agree with Altamont but really its up to your DM. If he feels it is in line with 7th level spells to change a chain shirt into Full Plate that is up to him. It's not like you are creating something from nothing, which is something a wish could do, but modifying an item into a different, but similar (and better) item. Seems like a grey area to me but I know there are many people out there who don't think Wish or Limited Wish should ever grant you any type of positive benefit if you stray from the options listed in the PHB without turning it into some kind of monkey paw wish.