You can install multiple versions of Sonar. It does not overwrite any existing install. Each version gets installed into it's own folder, so uninstalling the latest should be no problem.
The only problem I'm aware of is uninstalling an older version. Sometimes it has plugins that your existing projects may depend on, that are no longer supplied by the latest Sonar version. Certain FX and instruments came with X2, that are no longer supplied with Platinum, for example.
Many people leave their old versions installed side by side with the new to maintain plugins. The program folder uses relatively little disk space, so no problem. Or you can always re-run an old installer with custom install mode, and only select the plugins that you need to be installed.