When I heard the radio tech, he said something GEN 3 was back up?
You're still not giving us a
frequency. Asking when DEC "switched to P25" without providing any clue as to what frequency you actually heard DEC units operating in P25 mode on is not going to get you an answer to your question.
Hopefully k2hz will see this thread and shed some light on the situation. I've been working with him for the past few months confirming DEC PL tones in Western NY, and he has made absolutely no mention of any P25 activity on DEC's VHF frequencies. Since he's in Rochester also, and has been logging 24/7 for nearly two months, he certainly would've picked up on anything of the sort.
I suspect Steve is correct, and that you are hearing DEC on local Monroe County P25 frequencies, not on their own VHF system.