When you buy a Screenpresso PRO or ENTERPRISE license, you purchase a perpetual license for a one-time fee which includes one year of upgrades and maintenance (updates x.y). You can choose 2 or 3 years of maintenance if preferred.
After that, you can pay an
optional maintenance fee to retain support and upgrades.
This is explained in the FAQ section on the
pricing page.
At the end of the maintenance, you keep the version you have.
For instance, if your maintenance ends tomorrow you will keep your actual version, but when a new version is available you will get a warning telling you that a maintenance renewal is needed to upgrade. At that time, you will be able to choose to ignore the upgrade and remain on your current version forever.
All previous versions of Screenpresso are available
here, and this
page tells you which version of Screenpresso is working with your key.