To my knowledge, I don't think it is. If you already have a contract with Cellular One, you keep the contract until it runs out, then you'll either have to switch to AT&T, or find another provider. You also can switch to AT&T at anytime without contract penalties.
I just spoke with a AT&T Customer Service Supervisor. It appears CellularOne users who are on the cheaper plans ($50/$99) a month will be kept on a old legacy system until their contracts run out. This prevents CellularOne users to upgrade to a AT&T phone which offers data packages, or if their sim cards go bad good luck as AT&T says they dont offer CellularOne sim cards and will require customers to switch over to more expensive plans. This is highway robbery. Talk about REACH OUT AND TAKE ADVANTAGE OF US.
NO. you will not have to do this. people on cellular one will be able to stay on the cellular one service they already have. if you want an at&t rate plan and new phone, then you will need to sign a new contract. but you can stay with what you have as long as you are happy with no issues.