I tried that, even though the program never asked this before, the Parallels 14 key was enough. When I use the Parallels 14 activation key and Parallels 12 as a previous version key I get the following error:
The previous activation key has not been accepted by the license server.
Please make sure you have entered the key correctly. Your activation keys are stored
in your Parallels account. If you don't want to bother yourself with numerous activation keys and upgrade procedures in future, we recommend that you purchase a subscription to Parallels Desktop.