Well, different Q version (Q2 and Q3, for example) can be installed side-by-side and should work without interfering with each other. Still if you are not required to use previous versions it is best to first uninstall those versions and then install the latest ones.
We have just recently introduced automatic upgrade to our Silverlight/WPF installations for releases that belong to the same Q. This means that whenever a new release for a specific Q is available (for example a service pack) the installation will automatically remove the old one and replace it with the new version.
the Telerik team