How to change a payment method?

I have changed credit unions where ATT payments are to be drawn.   I talked with folks at ATT when I talked with all the other companies about changing, back in April of this year.  Att has run the draft for payment thru my older credit union in May & June, costing me $90.00 in fees, but paid the draft.    My older credit union will no longer extend that courtesy with or without a fee.   I need to change the credit union that Att will charge for my service.