I think you have to uninstall if you want to get rid of Poser 11.

However, you might want to keep Poser 11 for awhile. A lot of Python scripts won't run in Poser 12 or 13. Python 2 reached its end of life, so Bondware had to upgrade Poser to Python 3. But a lot of Poser Python scripts haven't been upgraded. Until you're sure you don't need any of those scripts, keep Poser 11.