Alexandr Hello. Simply cancel your existing subscription, wait until it ends and subscribe to the pro version.
Phil But do I just pay the difference or do I have to pay the whole subscription? I just did the standard subscription 3 days ago....
Alexandr Oh, you can contact Apple support directly to cancel your purchase and refund. Unfortunately we’re unable to do this.
Phil No, then I think I'll keep this version. Surely Apple will say that even they on their own initiative cannot refund the amount paid.