Some snags to consider: If the license has not been modified to include narrowband emissions, two things happen: 1) modification now flags the license to the FCC's enforcement bureau; 2) simply having the hardware in compliance and not doing anything with the license might invalidate the license. The emissions have to reflect what's being used. You can't go out and get DMR radios, for example, start using them in DMR and have only 20K0F3E (or 11K2F3E) emissions on the license without any other proper emission listed.
No one knows (probably not even the FCC) what will happen with the enforcement bureau getting a list of licenses. There are some licenses that are in queue for coordination or narrowbanding that have gone to coordinators. Those will have a grace period to allow for the last minute panic. There are licenses that have been granted waivers. There is no one place that lists who those are. So, only the licensee knows for sure. If they are like "deer in the headlights" over their license, they're probably in violation and low hanging fruit for enforcement (revenue!). If they are like "we sent our application in" they need to be compliant in their hardware, but the license gets a little grace period as the powers that be hack through the onslaught. If they got a waiver, they are somewhere in a sea of public notices.
Either way, we all woke up yesterday and everything turned on. Compliant or not. The honor system (or lack thereof) will prevail. In other words, if someone keeps operating and no one's caught up to them, but they're causing interference, someone may inevitably "drop a dime" on them and the consequences will probably be worse than not having complied in the first place. Notice I keep saying "probably" a lot. No one knows for sure.
P25 - It doesn't need to be P25. Minimally, it's any technology that fits one voicepath in a 12.5 kHz channelspace and fits a certain emission mask. P25 phase 1 and 2, DMR, NXDN, analog narrow, SSB, ACSB, OpenSky, reduced power TETRA, and maybe even full power TETRA all do that. It's also 9.6 kbps in a 12.5 kHz channelspace, or equivalent. So a 19.2 kbps data throughput can remain on a 20 kHz channel without modification or reduction in occupied bandwidth.