I called RTW desk and tried to extend mine until fall 2022 and the CSR said they weren't allowed to go past March 31, 2022. This aa.com web page has a date of March 31, 2022 for Covid related Flight Credit and Travel Vouchers and I believe that is where the date comes from.

When I asked the AA RTW desk what I could do, they suggested I contact Customer Relations. So I contacted Customer Relations via aa.com and told them I was doubtful Asia would be open by then and asked for an extension to the fall of 2022. They replied in a few days and notified me that they had extended the expiry date of my RTW ticket to December 31, 2022. To contact Customer Relations via aa.com follow the Contact American link at the bottom of their web page. Good luck.