Hi Andy,
While not strictly necessary, I recommend to uninstall before installing a new version. You have an Uninstall option in the PixInsight Platform item of the Start menu, so uninstalling is just a matter of a few seconds.
This will change when we implement an online update system. This will manage all the updates available in a completely automatic way.
Note that uninstalling PixInsight never touches your license file (.pixinsight-license on your personal folder), so you can unisntall and reinstall as many times as you want without problems. Of course, as long as you don't lose your .pixinsight-license file, you'll never have to reactivate your license.