said by jaminus :

Signals on the 850mhz band tend to work better for mobile users than signals on the 1900mhz band. While the output power for EDGE signals is the same on both bands, 1900mhz is a shorter wavelength and so the signal doesn't travel long distances from cell towers.

But on the flipside, 1900mhz signals penetrates buildings more easily than 850mhz signals, so mobile users in dense buildings with lots of metallic structural elements might actually notice better coverage thanks to the shift of EDGE from 850mhz to 1900mhz.