I believe you now have to add value (eg book a flight for more than the value of the ticket, and then cancel that).
I had this same question too. I booked a flight with a trip credit expiring in March. The remainder was issued as a new trip credit with expiration still in March. So if I book a new refundable flight before March that costs more than the trip credit, and then cancel it, I will get a new trip credit for the full amount with a new expiration date?