I don't know the exact method of switch over, but from what I've seen from my observations:
Both systems are online and have a link between them...similar to a separate site (but its a separate system to us).
My guess: if the admin "slaves" a talkgroup to a system, it will always be available there. Otherwise, the system watches for affiliations on a talkgroup before linking between the systems.
So you are correct on them (PD) being on both the EDACS and P25 system. If you happen to have two good radios, listen to the same talkgroup on both the EDACS and the P25 system...the dispatchers come through both at nearly the same time (their audio goes straight to the controllers), where the units you can tell which system they are on based on which system you hear the decoded audio on first. Even on the same talkgroup, you will occasionally hear someone on different systems. It seems most (if not all) of the PD is on P25 now. I think they did a phased radio re-program. Had a set of officers bring radios in on schedule, they were re-programmed (added new system, took out old one) and sent them back out. Once migrated, I haven't see any radios back on the old system.
Last I looked into it a lot, I didn't see many FD radios on the new system yet, so I don't think they have really started their main migration yet.
What did you mean by your last sentence/question? Where are you listening - on a scanner or the streaming stuff? You can listen to all (non-encrypted) traffic on the P25 system here:
Broadcastify