With a Personal or Business license subscription, you can always use the newest version of the program. The same is true for Free licenses.

By purchasing a one-time fee Personal or Business license, you get the latest officially released major version of eM Client, all the smaller updates related to that version, and all older versions.
If you decide to upgrade to a newer version of eM Client, you will need to renew your license.

If you purchase one-time fee license with Lifetime Upgrades, you will gain access to all present and future versions of eM Client. You can purchase Lifetime upgrades separately at any time.

The full model is described and explained here.