How do I upgrade from the standard version (which I bought) to the pro version on Mac?

Hello.
Simply cancel your existing subscription, wait until it ends and subscribe to the pro version.

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....

Oh, you can contact Apple support directly to cancel your purchase and refund. Unfortunately we’re unable to do this.

There is no way to pay the difference.

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.

They can and usually do. Just tell them that you want to upgrade.

I've never done that. Where should it be written?