Save As - Driving me crazy!

In my daily work I open a lot of older files modify them and save them as a new file. When opening a new file the first thing I do is do a save as and continue my changes thinking the file I’m working on is the new name I just gave it! But Rhino 5.0 just saves a copy of the file under the new name and you continue to be working on the original file? Why is this? Every other graphic program I use when I do a save as all your consecutive saves are on the new file?

This only happens when saving as an older version IIRC… or? --Mitch