Jackson will be re-purposing their old 800 mhz frequencies for this new system. Right now, they have them divided in half. Half on the old system, half on the new. When they get ready to bring the city online, they will use cross patches between the old and new systems while they convert subscriber units over.
Do a search, with your scanner, of the current Jackson 800 channels and you should find the new P-25 control channel transmitting, but there's probably not any traffic on it yet.
I'm not really sure what the three 450 mhz frequencies are being used for. They're licensed for 4 watts ERP mobile so they will not cover much area and they have a Mototrbo emission designator.
I do know they're planning a Mototrbo control station to link to a Sheriff's Dept. talk group on the new system so the city units can communicate with the county. I also think they're doing the same type link between the city and county fire departments.
I agree with the Medlink patch to the Hospital's UHF system. I have a friend employed by the hospital who uses a UHF portable and he can monitor Medink through the UHF system.