The U.S. Federal Communications Commission this week issued an Enforcement Bureau consent decree stating that AT&T has agreed to pay $700,000 to the U.S. Department of Treasury to “resolve complaints that the company switched certain consumers to its mandatory monthly wireless data plans even though it had promised they could retain their existing pay-as-you-go data plans.”

AT&T is also now required to refund excess charges that were paid by individual customers, which could be as much as $25 or $30 a month in the case of customers who didn’t use much data.

The fine ends an FCC Enforcement Bureau investigation that found AT&T forced certain customers, who were grandfathered in with pay-as-you-go data plans, into newer (and more expensive) monthly data plans. In September 2009 AT&T made monthly data plans mandatory for new subscribers, but told the grandfathered customers that they could keep their plans. In November 2009, AT&T began transferring these users, without their consent, to the same data plans that were offered to new subscribers.

“Today’s action sends a clear signal that wireless carriers can’t wrongfully charge consumers,’’ said Julius Genachowski, FCC chairman. “These strong FCC accountability measures will ensure customers are not over-charged. I am pleased that AT&T is taking the appropriate steps to resolve this issue.”

AT&T has also agreed to a compliance plan, which includes customer notifications, customer representative training, and compliance reports to the FCC.

In July of this year, AT&T followed Verizon by changing its subscription model once again. Though AT&T didn’t force customers from their monthly data plans to the new shared data plans, Verizon did end its grandfathered unlimited data plans, upsetting many customers. It was a blatant cash grab that left customers paying more for each GB of data, but Verizon was upfront about the matter, meaning the FCC is unlikely to issue a fine for its actions.