How can one activate the program monthly?

I’m not using Harmony very much right now. So I only want to activate it monthly.
How do I do that? I have two versions installed on my computer: Essential and Advanced.
Can I just enter a serial number or do I always have to download and install the program completely?

I have a perpetual license version however if I was in your position I would go to the Toon Boom website and go through the motions of subscribing monthly. If it is the same version you may receive all the data you need before actually downloading the software. You can try activating what you have with this data. If it does not work then just download the software acting as though you do not have anything installed.