Hello ralmac,
Thank you so much for using our theme WeblogPro. Free theme never directly upgrade to pro due to its additional features and functionality. So, first you need to add the theme manually, and after that, you can configure the some setting again. Your post, page and all other contents remain as it is, just may need to save the settings of theme Appearance > customize.
If you faced any problem, let us know. Please follow the theme documentation for more details.
Regards