Hello S.J.M van der Stee,
Thank you so much for using our theme Education Base Pro.
While upgrading from free to pro, only the settings need to set again, because of the added features on the theme, you should save the customizer and widgets settings again. Free theme can’t upgrade to pro on once. Please follow the theme docs, blog posts and videos to know about the settings. You can follow the same procedure on pro theme as on free theme. Don’t worry. your content won’t lost.
If you made any customization on theme, it will lost so we recommend you to work on child theme.