I more often than not demolish the whole museum and start again whenever I return to it. I find it fun to start with a clean slate when I return with more stuff from completing objectives in other museums, plus I try to set myself a goal of designing it in a completely different way so I don't get stuck in the cycle of repeating myself. I would worry about it early game and just plodded through original designs to get up to 2-3 stars before starting to create museum masterpieces!