T-Mobile has committed to maintaining the service plans of anyone who was a subscriber to either service as of February 2019, for the next three years as of this week (five years, in California). However, you should expect that those legacy service plans won't always survive a new phone purchase. Upgrading your phone means T-Mobile may ask you to upgrade to one of its current service plans; switching from an older 4G phone to a new 5G phone means it almost certainly will.

As part of the merger, T-Mobile is offering some inexpensive new service plans. You'll be able to get a 2GB data plan, plus unlimited talk and text, for $15, or a 5GB plan for $30.

The merged company has committed to not raising prices for three years, so look at Sprint's and T-Mobile's prices right now; that's what you'll pay on a new plan. Expect prices to go up after three more years.