My contract with my carrier consists of (roughly--there's lots of legalese in there) the following: Carrier provides A, B, and C services at X service level agreement, and I pay Carrier $YY for the privilege.
Nowhere in the contract I signed does it say that I give the carrier permission to change the services provided or add additional services without my express permission, nor does the contract say that they can charge me extra for any additional services that they may deem I 'require' at some point in the future. If you wish to make a unilateral change to my contract, I consider that a breach or a "material change of contract" that allows me to quit without penalty or ETF.
If the policy is "If you bring your own smart phone to our network and put our SIM in it, we will change your services and costs," that had damn well better be in the contract I originally signed, or it is immaterial to the agreement we have. End of story.