Both systems will be online until everyone has P25 radios and is on the P25 system. The new gen of radios will have both systems in them. When they upgraded the IDPS to P25, they had both up and running for an short time. Everyone had both the analog and P25 systems in their radios, and at least with IMPD, they switched one division at an time. Once everyone was on the P25, they kept the analog as backup until they made sure everything was working. Once all the users were on the P25, and things were working smoothly, they killed the analog operations. With SAFE-T, most users will be doing the same thing. The Statewide users, such as ISP, DNR, DHS etc, will use the P25 where it is available, and the legacy system where it is not. There will be patches etc set up where they are needed, and then once the full system is done they will use P25 full time Statewide. The main issues will be more localized in the County/Municipality arena. If users are properly trained, there will be few, if any hiccups. That is also what part of the delay is about. Training everyone on how the transition will happen, and how to operate during it. How to use the newer radios, and switch between both the legacy and P25 systems etc. Example: Lets say ISP D-33 is undergoing the transition. In the radios they will have both systems set up. For Dispatch there will be something like say - ISP DISP and ISP DISP* The one with the star will be the new P25 one. If they need to contact Dispatch and are in an area not yet P25, they would use the non starred TG. There will most likely be simulcasting on both systems until things get settled down.
Point being, there are many ways they can work out issues until the transition is complete. And then around 2020, you get to see more testing and such as TDMA (P25 Phase 2) rolls out.
The first things you should hear on the P25, will be range/coverage testing, and then some full time usage. For FD's there will be some transitional testing especially as they work out the best options for how to deal with P25 and SCBA's. Since P25 has been around a while now, they have found out the best "fixes" and "workarounds" for dealing with the audio issues with the SCBA's. Back when P25 first started being used, there was a lot of issues with that area. And also, expect most FD's to use Locution Dispatching and only the smallest agencies sticking with pure human vocal dispatching in the near future on the P25 system.