According to TMoNews, T-Mobile may announce the plan as soon as Monday, March 4th, and the plan could go into effect as soon as March 24th. Once the switch is flipped, T-Mo will kill all of its contract plans and subsidies, and start to move towards full price phone sales. Additionally, existing customers will not be able to re-up their existing contracts, and will have to move to a new plan. Early termination fees will also become a thing of the past.
And, if you're worried that you'll have to pay $500-600 up front for a new smartphone, T-Mo will have you covered there, and will continue their off-contract installment plan, and will add a new tier to the plan. The idea is that you'll pay about $99 up front for a new smartphone, then spread out the rest of the cost with monthly payments of about $25-30.
The rebranding may also encompass the way T-Mo refers to its 4G service by calling it "Dual 4G", which is supposed to refer to the combination of HSPA+ and LTE. Given that T-Mo HSPA+ can be faster than Verizon or AT&T LTE depending on where you are, this could be a pretty interesting move.