There’s nothing in the engine to stop that from happening. Plus, consider this–you download free and start working on a project to try it out, you decide to continue with it and buy a Pro license because you want those features, you can still release a game even though you did work with the free license before you bought Pro.
EDIT: To clarify, I’m saying their terms are inconsistent, that it’s encouraged to upgrade to Pro from free, but if they truly were preventing people from using a mix of pro and free licenses then that would mean you wouldn’t be able to do that. And if they wanted to actually stop people, they could easily allow people to do the upgrade but prevent the project files saved in a pro version from being used in a free version.