According to TmoNews and Howard Forums, T-Mobile is beginning the process of moving customers on some of its many, many grandfathered plans (allegedly, 350+ according to a commenter on TmoNews who pretty obviously works for T-Mobile) to unadvertised "Select Value" plans. This does not apply to customers on T-Mobile prepaid plans. T-Mobile issued official confirmation that the carrier would be forcing subscribers to migrate - that is, it's mandatory - but that customers unhappy with the change (after being notified) can cancel service before February 1st next year without paying any early termination fees. The forced migration should be completed by the end of the year. And yes, this also probably means the days of getting subsidized on-contract phones for T-Mobile through retailers like Costco or Wirefly are coming to an end, as it's pretty clear T-Mobile is funneling all new postpaid business into Simple Choice plans.
So, what happens if you have an old "Classic" plan or other postpaid service that's no longer offered? You'll be moved to what's called a "Select Value" plan. These plans are not advertised by T-Mobile, and only those customers pushed into them from an old grandfathered plan will ever be able to take advantage of the rates offered. That is to say, T-Mobile isn't stupid - they're attempting to make sure almost no one will actually end up paying more for less service on the new plans, and allegedly (according to that same commenter saying there are 350+ grandfathered plans) T-Mobile has put together 49 different Select Value pricing schemes to keep everyone happy.
However, you won't get to choose which plan you get - you'll receive a plan comparable to your existing service (to the extent possible), and that's what you get, period. The plan is chosen by T-Mobile, and if you don't like it, you can move up to a Simple Choice plan or cancel service. Most people should end up satisfied with the new plans, though - minutes are gone (all plans are unlimited), and most of the Select Value offerings will have unlimited texting, as well. If you were on an old unlimited data plan, you will keep that unlimited data. The few scenarios where customers might end up paying more involve those who have feature-stripped their service down to negotiated minute plans (100-200, for example) or use pay-as-you-go SMS.
What is unclear is what will happen with new lines added to a Select Value plan, though my guess is they'll be forced to Simple Choice instead.
Here's the statement from T-Mobile:
Maintaining thousands of rate plans is the norm in the industry, but we think it creates unnecessary complexity. Simple is better, which is why we’re reducing the number of older plans in our systems. We’re giving customers on these plans the opportunity to choose a plan that best meets their needs. For the vast majority, their plan will provide similar or better features at a comparable price.
This change will undoubtedly affect millions of T-Mobile customers, and probably result in a lot of confused and angry phone calls, even if most of those customers are probably getting a better deal at the end of the day.