Change the costume and you likely have to touch up all the animations, then run all future animations through the differences. Why a lot of side scrollers will eihter make a whole new character, or have one outfit max. It just slows it down, and this game does not need any more help with slow development (albeit high quality).
Really the gear makes no odds at all, unless you need to make the game easier (more hit points, more damage), but you can just make an easy mode for that. No need for level up or gear. But I suppose, maybe the backers want to dress up Onyx!