Uninstall 3.5 or not, when installing 3.6?

Both versions use the same authorisation so you won't have any issue with that. The only issue if you keep both versions installed is that you will have two pianoteq VST plugins (or AU or RTAS) in your VST folder (because the one of version 3.5 is named "Pianoteq35.dll", the one of version 3.6 is named "Pianoteq.dll". Most hosts will pick one and ignore the other, that's why we recommend that version 3.5 is uninstalled first. Future upgrade should be easier as we will stick with the "Pianoteq.dll" name.

I tried the uninstall route (it complained about 3.0 and 3.5 still being installed), but then that left me with hundreds of broken projects, including my template, because they were created with earlier versions of the plugin that my DAW no longer sees.  I have gone back and reinstalled 3.5 and put a copy of the 3.5 dll and renamed it pianoteq30.dll in the 3.0 folder.  Hopefully this is the last round of this problem.