From 15.3 Ultimate to 16pro?

Please see:

No one is ever forced to buy a copy. Choosing Zorin OS Pro is a means of supporting development, keeping the project going and getting some Goodies for doing so.
Some users choose to pick up a copy of Zorin OS Pro (Formerly Ultimate) in leaps and bounds - skipping a release in between.
Zorin OS Core and Zorin OS Lite provide a Free Operating System that allows anyone, anywhere in the world, access to use of their computer and everything needed to do so.