For me I have not uninstalled a Sonar to date. LOL. There is an advantage in not doing so. The new version can read the ini files from the old and transfer that information to the new. This can save a lot of setup time.
In my case I really only care about the master.ini file. It holds all the MIDI instrument ini info for various hardware synths I have. Key bindings will also be transfered.
When I went to win 8 I did so with a clean install which meant no old Sonars on the new HD. So I just copied over the needed files and replaced the new files and had no trouble getting my hardware synths working with all the patches and banks plus expansions cards that are on them.
I haven't had to setup Sonar for years because of this ability.
What the bottom line on this is you have a choice that has no wrong answer. However if you do have a somewhat complicated setup let Sonar migrate the infor from the old to the new. Or copy the files you may want to keep to a safe place then replace the new files with the old. These are the .ini files Sonar uses for setup and configuration.