Experienced a similar issue for a flight DEN-IAD on Monday 3/25, cancellations received for both myself and my companion this morning. Attempting to change online gave an error, so I called the 1-800 number, wait time was luckily only ~15m. I was able to snag the last 2 seats on a later nonstop the same day, worked out fairly well. Frustratingly I had originally been booked on the later flight but changed to earlier when they waived the fare difference as the later was showing operated by MAX and the earlier time was more convenient. Glad to be able to change back though, it's showing fully sold-out on the site now.

The rep I spoke to said that they were getting huge numbers of rebooking calls that day, seemed like they had loaded some sort of schedule adjustments/cancellations earlier today which resulted in all the email notices and ensuing activity.