I do not like doing fresh installs. It wastes hours of re-configuration just to get the setup how I like it, which is why I much prefer direct upgrades.
Is there some way to upgrade from 15 Pro to 17 Pro, without having to perform a fresh install and without having to pay for 16 Pro? Or is the only direct upgrade option available to pay for 16 Pro and direct upgrade, then pay for 17 Pro and direct upgrade?
Yes, you can upgrade to Zorin OS 17 Core or Pro at will. However, it is necessary that you first upgrade to Zorin OS 16. You do not need to upgrade to Zorin OS 16 Pro in order to upgrade to Zorin OS 17 Pro.
The Zorin Upgrader must begin with fully upgraded packages.
You can upgrade from 15 to 16 Core. Once that is completed, you can use the normal updater tool or terminal
sudo apt update and sudo apt full-upgrade
to upgrade everything to Zorin OS 16.3
When finished, launch the Zorin Upgrader where you can select the option of upgrading to Zorin OS 17 Core or Zorin OS 17 Pro.