I purchased Poser 12 but my current machine has Poser 11 on it. I don't need two versions on the system as I practically never go back to old scenes, but I'd like to know if there's any way to upgrade the current version in place instead of what I did when upgrading from 10 to 11, which was uninstalling and reinstalling practically everything. I had attempted to install over the previous installation, but that didn't quite work out like I had hoped.
So is uninstalling and reinstalling really the only good bet other than side by side installations? Or am I missing some way of telling 11 that 12 is sitting here and it's time to grow up?

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.

By reinstalling everything, are you talking about your runtimes? As randym says, most Python scripts don't work, unless you're familiar with Python and can do a little magic.

If it is the runtimes, you can just move those to a new location. I'm not sure if it will uninstall will remove personal files, but better safe than sorry. It won't uninstall external runtimes