[dogcow]

Pre-shared, static keys are unfortunately quite common. However, the P25 standard does provide for re-keying over they air through a process known as OTAR (Over The Air Re-keying).

To put it very simply, radios communicate with a central Key Management Facility (KMF) using a special key (UKEK, Unique Key Encryption Key) to securely transport the new key material. There's more to it than that, of course, but these features are heavily used by the feds and also by larger state and local systems -- because manually re-keying each radio is a huge pain.