benbacca37,
The Pro version is an unlock key, this means that the free version will see this key and unlock all Pro features. There are multiple reasons it works like this and why a lot of developers do it this way: Only one code base should be maintained. Easy to test as a developer, install key/remove key. You don't have to switch to another app, saved data gives no problem. and multiple other small advantages.
So just install the free version, then the pro key and the free version will become the pro version
PS: Thanks for your support!