The question is too large to easily answer, and difficult to boot: there are some resculpts that are "returns"...which is basically no changes at all (e.g. Flashman). Some resculpts have the most minor of changes (e.g. Spider). Many have a decent amount of changes (e.g. Awesome, Sunder, etc.). Some have what many might consider "too many" changes, like the Project Phoenix mechs, or the Dark Age mechs.
Then, there are resculpts of the same mech variant, and resculpts that depict a different variant but the same model (Komodo KIM-2, and Komodo KIM-2A for example).
It would help narrow it down if you could be more specific.