When you choose to upgrade to the PRO version to get rid of the ads, you will notice there are now two separate apps on your device. You can now easily transfer all your lists and settings from the free version into the pro version.
First, open the free version, and go to:
Settings -> Backup -> Export
Then open the PRO version, and go to:
Settings -> Backup -> Import
Here you navigate to the folder “ShoppingListBackup” and select the file you just created, usually the most recently created.