I don't have the rule book in front of me for the page number, but where it talks about equipment is says that if you cast an equipment spell for a spot that is already occupied, then the equipment you already have out goes back into your spell book. This means that you can swap out your equipment without having to worry about losing it, unless your opponent dissolves it of course.
In other words, just cast the spell and replace what's already there, putting it back in your spell book.