AT&T Inc. said Monday that if its deal to buy T-Mobile USA goes through, T-Mobile subscribers with “3G” phones will need to replace those to keep their wireless broadband service working, but there will be plenty of time to do that.

Dallas-based AT&T said Sunday said it had agreed to buy T-Mobile USA for $39 billion. If approved by regulators, the deal would close about a year from now.

AT&T said that some time after the closing, it plans to rearrange how T-Mobile’s cell towers work. The spectrum they use for third-generation services, or 3G, will be repurposed for 4G, which is faster.

That would leave current T-Mobile phones without 3G. They would need to be replaced with phones that use AT&T’s 3G frequencies. Ralph de la Vega, AT&T’s head of wireless and consumer services, said this will happen as part of the normal phone upgrade process.

“There’s nothing for them to worry about … it will be done over time, in a way that’s good for customers and good for AT&T,” de la Vega said in an interview.

The whole process will take several years, he said.

AT&T shares rose 27 cents to $28.21 in midday trading Monday.