YES, it is s/d first then c/d for life. Mine are all on c/d. they eat the dry, but the wet might be a good choice in Luna's case. Or mix them together so she still gets the "crunch" of the dry food.
Has Luna had a complete blood count work-up? Constipation can definitely be caused by teh hairballs, but it could be more serious as well, Maybe when you ask about the canned food you should ask their opinion on the CBC.