AT&T's terms of service includes a section on off-network usage. They place a monthly upper limit on the amount of domestic roaming airtime minutes (40% of your Anytime minutes to a maximum of 750 minutes), or domestic roaming data usage (20% of your regular limit up to a maximum of 24 MB) or domestic roaming text messages (50% of your text messaging limit to a maximum of 3000 messages).
If you exceed these upper limits, AT&T will contact you to inform you of the fact.
If, after being informed of the fact, you continue to consistently exceed the upper limits, AT&T may, at its option, convert your service to one that has a surcharge on domestic roaming, or prohibit you from connecting to the network at all whilst you are roaming domestically, or suspend your account.
Upon being informed of the fact that AT&T is about to make any such changes to your account, if you do not agree to the change, you may terminate the service immediately without penalty.
All this deals with off-network domestic roaming (ie traveling to another area inside the USA where AT&T doesn't own its own infrastructure), though, and it deals with somebody who's continued to use their service after they move; not with somebody who's planning to leaving the country entirely and hasn't even started doing any roaming.