In most cases, in order to transmit legally on a frequency band of the
sort you're discussing, you must be licensed for that frequency _and_
you must use a radio which is "certificated" (formerly "type
accepted") for use on that frequency band.

The certification is, in effect, a statement by the radio's
manufacturer that it is compliant with the FCC regulations for that
band - maximum power output, frequency accuracy, spurious emissions,
and so forth.

Amateur radio transmitters are an exception to the rule. They don't
have to be certificated, because the amateur radio service is intended
to support experimental and home-brew use, and because the (licensed)
operator has accepted legal responsibility for not transmitting in
ways which break the regulations.

So... if you reprogram a commercial or public-safety radio onto a
frequency band outside of what it was originally certificated for, or
if you physically modify it, you're likely to void its certification,
and it might not be legal to use it either on the original frequencies
or on the new ones. You could reprogram it for amateur-radio use,
though, and that'd be legal as long as its emissions met the standards.

Thanks for the info. That's something I wasn't aware of. No wonder I
wasn't seeing a lot of 'answers'!