Digital is a different beast compared to analog. It's not always as easy as just swapping out the new digital repeaters for the old analog ones.
Coverage is going to appear to be slightly different. Digital works great up until the point it doesn't, then it just sort of falls off a cliff. Analog was nice because it would slowly degrade, and the human brain can often fill in enough of a garbled analog transmission to make it work. Digital, like I said, works then falls off the cliff.
Legality issues aren't too bad. Emission designators would need to be changed on the license(s). If additional repeaters are required to get the coverage they need, then those sites need to be licensed also.
All your gear will need to be replaced unless it is already capable of handling the digital emission that's chosen.
Really, and I can stress this enough, they should be looking at turning up the new digital equipment in such a way as to retain at least some of the analog system. While digital is nice, when it fails, it fails hard. A lot of agencies buy the new gear, tear out the old stuff and then find out later that a failure has completely hosed all their communications. Keeping at least one analog channel for interoperability with other agencies that are not digital, and to serve as a back up system is really a good idea.
Most importantly, the agency should really consider getting an outside consultant to help research, plan and execute the transition. Where I've seen things fail is when the agency relies solely on a single shop/vendor for the entire project. This is kind of dangerous. A neutral consultant can save a lot of problems.
Other issue is "brand loyalty". This is common on the law enforcement side. "Chief Bob, who's been with the agency 40 years has always carried a brand-X radio, so that's all he'll consider". This can be an issue since SOME brands like to sell expensive and over provisioned radio gear to small agencies that don't have the staff or a consultant to analyze their actual needs. I really hate when I see a small agency that did fine with a single or two channel VHF system suddenly get forced into a 700MHz trunked system with top tier radios, all in the name of "interoperability". There are vendors and manufacturers that will happily drain your county coffers of every single last cent of taxpayer dollars.
Consider long term costs of ownership, including maintenance contracts. Consider a 10 year replacement cycle and figure in the costs of that.
Don't fall for the sales tactic of "if you really support your law enforcement, you'll buy them the best". That's a load of crap.
If repeaters are being replaced, consider the rest of the system too. Coax can deteriorate over time, antennas can, too. It makes sense to use the system downtime to replace these things. It might prevent a failure down the road.
Make 100% damn sure that when they program the new radios that they put EVERY SINGLE LAST ONE of the nationwide interoperability channels in the radio. This should be mandatory and any guy who programs radios for public safety that ignores this needs to be drawn and quartered.
Simplex channel should be mandatory, also. The old say if "you fail to plan, you plan to fail" applies here. Assuming that the new radio system will never fail work fine, until it fails.
