tlawhon
I believe the only difference is that Save As will leave you with the new project open, and Save Copy As will leave the original one open.
That is correct, if you "Save As" you are now working on the newly saved project. If you use "Save Copy As" you are working on the original project but have saved a new one as well, that's closed.
Very handy for saving landmark projects, such as "My Project-Tracking Finished" and carrying on working on "My Project".
It may be small but I am finding that one of the highlights of Sonar. Does that make me sad?