Cannot upgrade from 16.3 Pro to 17 without purchase new OS

Users can go from 16 Pro to 17 Core, but currently, they must do a fresh install.

This is a pretty fair question, though. It seems like the Upgrader should offer the ability to go from Pro to Core should a user wish to or need to skip a purchase cycle and use the free version.

However, in looking over the Upgrader Source Code, it becomes pretty clear why it is not offered in the Upgrader Tool. In order for the Upgrader to properly work without breakage, it must be able to address and upgrade existing packages and repositories.
For users who do not have Pro and are upgrading Core, it is a simple matter to add a repository and install software.
But for a user who already has Pro and wants to move to the next Core release, repositories and software (and configuration files) would have to be removed creating a complex situation that can easily result in breakage.

So, I believe that the reason the Zorin Upgrader tries to keep users in line with what they already have is because the Upgrader needs to do this in order to work properly, not to keep users on Pro.