@gparisot: All your customization should be retained after you upgrade to the pro version but to be on the safe side you can backup your customizer settings using Catch Import Export Plugin. Install and Activate the plugin.
Activate Decree free theme and export your customizer settings from the Catch Import/Export tab in the customizer. Now activate Decree Pro theme and import the same file. This should do it.
Let me know if this helps you out!
Kind Regards,
Skandha