As a data point, I have family that was booked on the original 6/21 flight.
AS rebooked them on a United flight for 6/22 through Denver without prompting.
After checking, no flight options were available on United to get to IAH on 6/21.
AS was operating flights to Austin, San Antonio, and Dallas with space for 6/21 so I am assuming they could have been accommodated in those if they wished.