I am not sure where the license file is stored, but to answer your second question, PGP has installed files in several locations around my system. (OS X 10.5.8, PGP 9.x)
First, back up your PGP keychains, and any virtual disks you may have. Then I would delete the following files.
Once you have deleted those files, you should be able to run the installer for your new version of PGP, enter your license info, and you should be good to go.