Those would be two different licences as your perpetual will remain as is and has nothing to do with your monthly subscription version.
If your perpetual is at version 2019.3.3 but pay for a monthly you'll always get access to the latest version through your subscription which is a separate license, ie. 2020.2.2 as of today, but your perpetual will never change so if you discontinued your monthly subscription you can go back to using your 2019.3.3 version.
Keep in mind any projects you make in a newer version will not be backwards compatible with older versions.