Not sure where people's AT&T bills are but after ordering a 6S through Apple I called AT&T and went to retentions to ask if I'd have any fees to switch carriers. They promptly offered me the 10GB plan for $80/mo or the 6GB plan for $60/mo plus line fees.
Might be worth a call if you don't need the current 15GB.
Might be worth a call if you don't need the current 15GB.