I can't speak for every department, but the county I live in has a policy of keeping enough of "last year's model" radios on hand, which still work fine in the system's current configuration, to equip absolutely everybody with a radio who might need one. They keep this reserve of radios well maintained. We're a Harris county (unforunately....) but while the current issue radios are all P7300s and XG75s for the most part, they've got plenty of P7100s to hand out like candy if they've got to call out everybody. They even made sure to have those radios in the emergency cache upgraded to P25 Phase 1 trunking. This cache of spare radios will remain useful until the county system switches over to Phase II, but even then, I believe the system will be able to accomodate Phase I radios for interoperability purposes.
This seems to me to be a very smart way for an agency to plan for emergencies. Retain enough usable retired equipment to support any emergency.