@fabian-huesser: If you had made changes to the theme files without creating a child theme then all the customization will be lost when you upgrade to the pro version.
If you used Catch Evolution Free as a child theme and have upgraded to the pro version all you need to do is get inside your child theme folder open style.css file and change Template:*text*
to catch-evolution-pro
or the name of the folder where your pro theme exists. This should bring back all your customization you made on your child theme.
Let me know if the issue still persist.
Regards
Skandha