Legacy plans will be terminated and subscribers will be moved to comparable current plans, Engadget reported Tuesday.
Existing customers reportedly may be offered special rates not available to newer subscribers, with prices beginning at $20.
T-Mobile's cheapest current individual offering -- its Simple Choice Plan --costs $50 a month.
T-Mobile issued a statement, saying it was "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," the company said.
In March, T-Mobile announced a major change to its plan structure, eliminating device subsidies but offering unlimited data, voice and texting at reduced rates.
The end of grandfathered plans would take place Nov. 1, T-Mobile said.