Can't they run a newer encrypted system on separate channels than the older system is using, with some kind of repeater set up between the two?
You'd need to upgrade at the base station upfront to have both the old and new systems, and add a repeater, but you could then take your time replacing the radios in the field instead of having to replace them all at once.
>>tzs+(OP)
Sure, but as long as a single unencrypted bridge is running, anyone else within range of that bridge can listen, so you won’t really get significant benefits of encryption until the switchover is complete.