I want to make way for new versions. May I, or is it a good idea NOT to uninstall (Sonar 8.5) older versions?

If you're going to do it (although it saves little space) do it before you install any newer versions.

Thanks, Mike!

When you install newer versions the installer checks to see what is already there and will use your existing settings where possible. So files that are the same between versions will end up in the same folders being shared.
If you then un-install an old version it will/may remove those files, rendering your new version broken.
 
The bulk of any Sonar version is the added content, plugins, softsynths, samples, etc. which will be installed automatically to the correct place if you keep your old version alive.  (Yeah, I know about the installer problems).
The size of the Sonar program by comparison is tiny and you get more by keeping old versions around than you gain in disk space by removing them.
 

If you have a 32-bit version hanging around, and you're thinking of exclusively using 64 bit in future, maybe keep a 32-bit Sonar around in case you come across an old project or a plugin that you can't get to work under 64 bit.