I would expect that the UHF frequencies may be used for other purposes, at least for awhile. I would expect that they would eventually be phased out completely.
As far as coverage outside of the city, it should be pretty good. With 10 separate tower sites there shouldn't be too much trouble recieving it in the southern half of Oakland and Macomb counties. Coverage should be pretty good for all of Wayne county (at least for monitoring), and probably a good portion of Washtenaw county.
The Detroit system is also a sub-system of the state's radio system, so if a Detroit PD vehicle should travel outside the coverage of the Detroit system, they will automatically switch to the state towers anywhere in the state.
Mike