Upgrading 1 of 2 LTE phones. Will 3G phone Still work?

[Pesto]

The upgrades arrived!  But, I am concerned if I activate one of the two phones, I will foul up the service for the second line.

We are actually upgrading both lines from 3G to 4G/LTE, but one phone is on the road and needs to remain in service, as is, until it returns home. Can I switch/activate one phone that just arrived to the 4G/LTE plan without disrupting service to the 3G phone on the road?

[mdram4x4]

yes