I didn't see anything in the docs folder about what to do for upgrading. Is there anything special I need to do?
There's a whole section about upgrading.
http://coppermine-gallery.net/demo/cpg14x/docs/index.htm#upgrade
I mean for pnCPG, not Coppermine.
Not really, simply overwrite all files. That should be enough.
In case you have copied some opf the blocks of the3.1 version into the PN blocks directory, I would advice to completely remove version 3.1 and do a fresh install of 3.2
Cas