Now, this is going to be controversial because "it's always been like that", and I can see that suddenly reversing the behaviour could confuse established users.
But... how many times have you done Save As (sorry, Save A Copy) then proceeded to start carving up the model - before realising that you're changing the old model, not the copy you've just created?
If reversing the behaviour is a bad idea, how about either introducing Save As in addition to Save A Copy; or adding a dialogue box to ask which file you want to keep open?
Similarly, Backup shouldn't leave you working in the backup copy IMO - if I'm creating a backup it's because I want to keep a record of where I was, not start changing it.
