UV Mapping - Possibility to keep average Scale through several Unwrap processes?

If I have my UVs sorted out but have to Unwrap one part again, it either mix up the whole map, or if pinned the scale becomes completely off. If I re-average the Scale, the UVs Overlap each other. Is there anyway to keep the average Scale of all UVs when unwrapping a small part without impact of the other UVs?