You don’t need to transfer it. You just have to go to the counter with the original boarding passes & pay with your credit card. I have upgraded my DS & DDIL who weren’t even on my itinerary. So basically the first 4 times you purchase upgraded boarding you will get the price refunded on your CC statement, regardless of who they were for.
Also, Note: the upgraded boarding only covers one flight. We had a connecting flight & the upgraded boarding did not cover the second leg of the trip. The agent said they can only upgrade for a flight from their airport, so they can’t do it for the connecting flight, even if you are willing to pay for both. You would have to check again at the connecting airport. So to me, this works best for nonstop/ direct flights.