No first hand experience here, but what I've read and heard suggests that replacement of main structural elements (e.g. wing spars) is pretty rare on the BUFF, but they've all been 're-skinned', at least the wings (and given the amount of readily visible 'oil canning' of the fuselage, I'd guess fuselage skin replacements are common as well).
I once did some inspections on a high time 737 going through a 'D' check. There wasn't much there aside from primary structure (although at least in the case of the 737 the original parts would, for the most part, be used when it was put back together).