Have a Sprint 5G phone? T-Mobile will force some of you to upgrade.

When T-Mobile and Sprint announced their merger, one of the main upsides touted for the "new" T-Mobile was its ability to build a better 5G network.

For 5G phone owners and eager early adopters, that sounded great. But now we know the newly combined telco's 5G footprint doesn’t exactly work in the favor of those who currently own a Sprint 5G handset.

Wireless customers with Sprint's 2019 5G phones — like the Samsung Galaxy S10 5G, OnePlus 7 Pro 5G, and the LG V50 ThinQ 5G — will need to upgrade their phones, according to T-Mobile, when the company eventually switches Sprint’s 5G network over to its own.

T-Mobile plans on informing customers with those phone models about upgrade deals. Until then, these customers will be able to continue using Sprint’s 5G network.

Galaxy S20 owners, however, are in luck. Of Sprint’s 5G offerings, that Samsung phone happens to already be compatible with T-Mobile’s 5G network and no upgrade will be needed. This includes S20 premium models like the Ultra 5G and Plus 5G, as well.

T-Mobile says that these Sprint Galaxy S20 customers will be able to start using its 5G network later this month.

As for the other Sprint phones, the lack of compatibility rests on how Sprint built out its 5G network. The OnePlus 7 Pro 5G and LG V50 ThinQ 5G do not have modems that support T-Mobile's 600MHz-based low-band 5G network. The Galaxy S10 5G, which T-Mobile initially promoted as being compatible before making a reversal, cannot register the LTE anchor band for T-Mobile’s 2.5GHz 5G.

The process of merging Sprint’s infrastructure into T-Mobile’s network is planned to take about 3 years. And the company has already begun the process in some U.S. cities. T-Mobile’s new 2.5GHz mid-band spectrum was just rolled out in Philadelphia. Next up: converting Sprint’s 5G to T-Mobile’s network in New York City.