Regarding what to do with it, you could remake the bottom "engine pod" and the aft section around the main engines. Make them look "sharper", like the "chin" on the bow. Also, add the "corridor" between front and aft sections. Also, the engine nozzles themselves could use remodeling. Overall, the part around the connection could use some slight modification. Right now, there's clear contrast between the old and the new model, but it's only obvious if you know what to look for. The problem with Deimos is that the aft is one big slab with pretty much the only place for detail being the engine cavity.