Here's a good explanation of the changes from P25 I to P25 II. There's a pretty good chance, as long as they don't switch to TDMA and stay with FDMA modulation, that the CPU and DSP in the 500 could be firmware upgraded to handle the new spec.
Here's the spec explanation:
Phase 2 radio systems will achieve one voice channel or a minimum
4800 bps data channel per 6.25 kHz bandwidth effi ciency. The P25
Phase 2 FDMA solution is fi nalized (CQPSK), and an alternate TDMA
solution is currently under development. Phase 2 implementation
achieves the goal of improved spectrum utilization. Also being
stressed are such features as interoperability with legacy equipment,
interfacing between repeaters and other sub-systems, roaming
capacity and spectral efficiency/channel reuse."