This is the current policy:
https://www.southwest.com/Coronavirus/
Midway down on the page it says:
"However, in recognition of the current travel environment, we are extending the expiration date of some travel funds:
-Customers’ funds that have expired or will expire between March 1, 2020 and September 7, 2020, will now expire September 7, 2022.
-Any travel funds created because you cancel a flight between March 1 – September 7, 2020, will expire September 7, 2022."
If you book a flight and cancel it now, prior to September 7th, travel funds will be created and thus will expire 9/7/22. I mentioned up-thread that I had to do that earlier this week for the purpose of combining multiple travel funds from my cancelled April trip. When I canceled my new booking to combine the funds, the travel funds did show a 5/27/20 expiration date, which is one year from the date I booked it. That's the default. However, the travel fund expiration date updated to 9/7/22 in accordance with this policy during the overnight hours, and the updated expiration date was visible to me when logged into my RR account on the next business day.
If you have any remaining travel funds from your cancelled trip, you can check the expiration date when logged into your RR account. If it still shows 5/28/21, I would call about it because that shouldn't be the case. For your December trip, you'd have to cancel by 9/7/20 in order to retain a 9/7/22 expiration date, otherwise it would default back to 5/28/21.