All the 600 Mhz sites in my area were already part of a very dense network, so coverage was never an issue. It seems like every 600 Mhz upgrade was for capacity issues since T-Mobile was also deploying the 5x5 block they had in the AWS-3 band.
The X55 modem can do VoNR (Done on T-Mobile with a OnePlus 8). I would expect T-Mobile to add VoNR via software update to all X55 phones once they have SA and VoNR ready on the network side.
They are trying to get two different results with the upgrades. The 600 Mhz upgrade was to get more range from the site to cover fringe areas. The 2500 upgrade is to get faster speeds & capacity in major population areas.