I called and did it over the phone with T-Mobile. 22 minutes of phone time - they said between 3 and 24 hours. It went smoother than I expected. Poor T-Mobile is so swamped that I got someone from their finance department.

FYI, they asked for:
a) ATT SIM number
b) My T-Mobile phone's IMEI number
c) Both account numbers from ATT and T-Mobile.
d) My ATT phone number.