It's always better to uninstall first, for the very simply reason that, in case a new version of the scenery do not need a file that was included in the oldest, by simply running the installer, that file will not be removed.
Now, I'm not sure if this is the case for KLAS, probably not, but as a general rule, that is valid with any software where the new version comes as a full installer and not as an upgrade program (usually, the upgrader "knows" what to do, and if a file needs to be removed, it will), is always best to uninstall first.
Of course, by Uninstalling and Reinstalling on the same machine, you don't need to worry at all about anything related to activation or reinstallation, the activation is never removed by the uninstaller so, the scenery will always stay activated.
