also if this is for a trunked system that you are allowed to be on, you will have to get the permission of the system manager and let him program it. hacking the radio/software to program it without a key is not advisable, especially when they know you have a personal radio on it!
as to rebanded systems, it depends on the new freqs. if they still have the original FCC channel ID then they work fine. i have never tried one on a system with the new channels though, nor researched it. but they should work even on them since they were still in production up until a month or so ago. they will not work on any freqs 861mhz or higher though.