They promise not to increase the rate on your current plan and get around this by just changing the plan you're on vs increasing the rate of your current plan.
> Price Lock is our guarantee that we won’t raise the price of your qualifying rate plan for new accounts. You can rest assured that T-Mobile won’t raise the price of your regular monthly rate plan price for current unlimited talk, text, and data on our network on Go5G Plus, Go5G Next, Go5G, Base Essentials, Essential Savings, Essentials, Magenta, MAX, 55+, Military, First Responder, Unlimited and Lite Home Internet, and Business Unlimited plans as long as you’re a T-Mobile customer and keep your plan. And customers don’t have to do anything to get in on this – everyone who activates after April 28, 2022, with an eligible plan gets Price Lock.
https://www.t-mobile.com/support/account/price-lock
> Price Lock is our guarantee that we won’t raise the price of your qualifying rate plan for new accounts. You can rest assured that T-Mobile won’t raise the price of your regular monthly rate plan price for current unlimited talk, text, and data on our network on Go5G Plus, Go5G Next, Go5G, Base Essentials, Essential Savings, Essentials, Magenta, MAX, 55+, Military, First Responder, Unlimited and Lite Home Internet, and Business Unlimited plans as long as you’re a T-Mobile customer and keep your plan. And customers don’t have to do anything to get in on this – everyone who activates after April 28, 2022, with an eligible plan gets Price Lock.