How much would it alter things if the Marbles were all replaced with exact replicas at the British Museum?

What would be the point in doing that? There are lots of replicas in museums but they tend to get labelled as such and they serve a purpose beyond just looking at them. The truth is the British public don’t really care about the Marbles, so if they went back to Greece I don’t think the museum would really need to put up a replacement.

Look at York Museum practically everything in there is replica, it’s all for the purpose of demonstrating history.