The moves would all take place after AT&T's purchase deal of T-Mobile USA is closed -- that is, if it's approved by federal regulators, according to the Associated Press.
On Sunday, AT&T announced it had agreed to buy T-Mobile USA from it's German parent company, Deutsche Telekom, for $39 billion.
If AT&T is allowed to buy T-Mobile, the Dallas-based telecommunications giant would grow from its current 95.5 million subscribers to about 130 million. Verizon Wireless currently has about 94 million customers.
As T-Mobile's 3G towers are reconfigured for 4G service, T-Mobile 3G users would have to switch to AT&T 3G phones that run on AT&T's 3G frequencies emitted by AT&T's 3G towers, the AP said.
For T-Mobile subscribers in the U.S., the effect won't be too noticeable, Ralph de la Vega, AT&T's head of wireless and consumer services, told the AP.
"There's nothing for them to worry about," De la Vega said of T-Mobile customers. "It will be done over time, in a way that's good for customers and good for AT&T."
The process of switching all T-Mobile 3G users to AT&T 3G would take several years, De la Vega told the AP.