I saw this notice today, and I think (but don't 100% know for sure) that this is in relation to an impeding launch of ATSC 3.0 broadcasting in our market.
Sinclair has partnered with Nexstar to carry each other's regular ATSC 1.0 broadcasts, where necessary, in markets where one or both is launching new ATSC 3.0 broadcasts. In our market, WKRG and WFNA are also asking people to re-scan on the 25th of August, so I have to believe that one of the companies' broadcasts are moving to other channels.
With Sinclair operating four licenses in our market, they certainly have the room to toy around with channel placement, especially considering how few subchannels are carried on most stations in our market. My guess is that WPMI and WJTC's subchannels will get offloaded somewhere else, and one or both of those RF channels will be re-purposed to ATSC 3.0 broadcasting. Mobile-Pensacola IS listed as one of the "First Broadcasts" markets for this new technology. Sinclair also already has ATSC 3.0 on the air in Las Vegas and possibly somewhere else by now.
From what I've learned, ATSC 3.0's efficiencies will allow stations to carry multiple high quality HD subchannels, or 4k/HDR broadcasts, plus interactive services. NBC was one network that was specifically mentioned in one article I read, saying they currently send HDR content out on their existing network feed, so they should benefit right away from an ATSC 3.0 transmission since it would be able to decode that extra data and pass it along to viewers without the need for a new network feed or anything.
One thing that's frustrated me about this is there is very little in the way of news articles or FCC filings about this stuff. Some stations do have filings on hand but not through the usual channels that I use to find this stuff out. But there's nothing at all so far about anything in our market, so this is all just speculation for the moment.