SOLUTION FOUND!
You need to ensure that there is no uninstall key entry for it - for example on 64 Bit Win8 = HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall{1A7638A1-E022-4F99-ADF3-F46DB04689C1} — you may also need to delete any keys for Plex from HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products.
Found it somewhere in forums (person who wrote this, thank you VERY much!)
Im now on the latest plex pass version 
