Hi Julia,
The upgrade to the free version is a standalone pro version. You’ll need to uninstall the free version, and then install the pro version. You’ll want to make sure on the settings page that you have checked off to save your icons, so they aren’t uninstalled when the plugin is uninstalled.
Once you install the pro version it will function exactly the same as the free version with additional toggles on the icon generator pop up and unlimited icons allowed to be uploaded.
If you are still having issues feel free to send me an email on the support page within the plugin (pro) or through the contact form on my site. I’d be more than happy to help out.
Happy holidays!
Evan