What happens if you want to change to at&t when ur 2 yr. ACS contract isn't over yet?

well i REALLY want a samsung eternity from at&t and i am willing to pay for it. but the plans look cheaper if u go with a family plan. cause i was thinking of convincing my mom to get a family plan with me. cause i plan on texting more than talking, so getting an individual plan for $40 would not make sense, so i thought getting a family plan would be the better way to go. but my mom has ACS, and her 2 yr. contract is not over yet. what will happen if she agrees to switch to at&t to get a family plan with me? will she have to pay a bunch of money to ACS or something?? ok thanks!!^_^