I have recently purchased and successfully installed a PSE2021 upgrade to my PSE2015 and it all appears to be working perfectly. My question is can I uninstall 2015 and remove it from my hard drive, or is it still needed for the upgrade to work?
Yes, you can remove it, if your catalogue stuff was imported correctly. Each version is a unique installation, having nothing to so with other installations (except for importing the catalogue). You might want to leave the older version installed if you thing you might use it to help others that have the same version installed.