Promise to Pay date cannot be changed

I am slowly cancelling and in some cases moving lines away from AT&T and moving them to my T-mobile account. I did a payment arrangement for 7/13 as that was the latest date it would allow me to select. I cannot make that payment until the 15th. Ive tried reaching out to ATT via X and they have been no help. I asked earlier on the att website and they are saying the date cannot be changed. But they also couldn't answer when service would be suspended.

Has any one else gone through something similar? I know is an account by account case but some general info would be great