This is for anyone booked on a cash reservation through WDTC with check-in dates from June 5 - June 21. They can modify to July 11 - December 31, 2020 to anything available at Ft. W and DVC resorts only.
Edited to add: I have zero clue how DVC is handling guests booked through DVC.
They e-mailed guests with reservations through from June 14 - June 21 last night letting them know they could try to start modifying this morning. (There were reports of this on the Resorts thread)
Room-only's should have been able to try to modify this morning as well. The e-mail sent out to guests stated, "You have the option to modify or cancel your Disney Resort hotel reservation or Walt Disney Travel Company package"
Tons of TAs reporting on FB. I'm getting my info from a TA friend who modified reservations this morning. If they continue with the rolling notifications, perhaps you will be notified via e-mail next Thursday at some point. That's just a guess, though, based on the rolling e-mails they have been sending out almost weekly (they skipped last week).