T-Mobile's "uncarrier" shift was that it would happen "as soon as" March 24th, so the date links up fairly well. The idea behind the shift is that T-Mobile is planning to kill contracts and subsidies. That would allow T-Mobile to offer its off-contract plans to everyone, and those plans are quite a bit cheaper. T-Mobile would still offer installment plans for those who don't want to pay full price on hardware right away.
It seems like we'll find out more soon enough, and we'll bring you all the info as we get it.