I have never built wheels so any help would be great.
Take the whole lot down to an LBS and get them to do it for you.
Failing that. As long as the wheels have an ERD within 1-2mm of each other, and the spokes haven't had too hard a life, then it's perfectly possible to re-use spokes. (buy a pack anyway, just in case when you undo them them they turn out to be worse than you thought, and you need to replace one or two.)
I have some Hope hoops, DT Swiss EX5.1d rims bulit onto Hope Pro2, and am thinking of changing the rims over to Mavic xc717? Can you use the same spokes or will I need new ones? Was thinking I could swap the rims over and then get my LBS to true them up propely? I have never built wheels so any help would be great.