You will not lose any content if you keep your purchased and downloaded content in a separate external runtime away from the prime install.  The only issue will be your Python Scripts which reside in the Primary install.  Those can be moved easily because they are contained within a single folder.  However, most of those scripts may or will not work in Poser 13 because of the Python Script transition from version 2 to 3.  Uninstalling the older Poser versions will not affect any additional content you physically installed, just the program itself.  If you are skeptical, make a copy of the prime runtime folder and keep it in a safe place.