Seems like most people in this thread know what’s going on but to clarify a few misstatements I’d seen:
AT&T is not switching to a new network - AT&T like all U.S. carriers operates legacy 3G networks and various bands of 4G networks. The 4G LTE network AT&T operates goes back probably 10 years or more, so there is not a “new” network being switched on, rather they will no longer be utilizing the spectrum used by the 3G network. This is not an AT&T thing for what it is worth, all the major U.S. carriers are sunsetting their 3G networks and repurposing them. Verizon and Sprint/T-Mobile also plan to complete this process in 2022, same as AT&T.
If you phone has a 4G LTE modem it likely will still be able to connect - the app that was posted is a great way for lay people to check, but you can also look at the specs of your phone to see what sort of cellular modems it has. The mentioned Galaxy J series is old enough that it’s in a “trouble” spot, some models of the J series actually shipped with only 2G/3G modems and no 4G LTE.
You may still be screwed when 3G goes away - Depending on where you most normally use your phone, even if your phone says it will work on AT&T’s 4G network (or any carrier’s), and even if you’ve run the app saying it works, you may still be screwed when 3G goes away. Why? A couple reasons. Some networks have gaps in their 4G LTE coverage such that you might regularly use your phone in an area where your connectivity is mostly on 3G, occasionally connecting some to the 4G network. I have some rural property I spend time at where my cellular connection is primarily 3G, when this switchover occurs I most likely will not be able to use my cell phone there without a mini-tower or something of that nature. For something like 95% of the country where people live, this won’t happen, but in some rural spots it absolutely will. The 3G network still covers more geographic areas than the 4G/LTE network.
There is no indication AT&T is planning to block phones via IMEI as part of this change - AT&T’s current business practice is to allow you to “Bring Your Own Device.” There are two-fold reasons for this. One, is back under the Obama administration there was a lot of talk of actually mandating carriers allow you to BYOD, at the time all the major carriers did allow it, but some of the niche discount carriers had weird setups where they were basically fully integrated with specific models of phone, and you couldn’t BYOD at all. No regulation was ever promulgated mandating it, but partially to head off regulatory oversight the majors have all allowed you to BYOD for like 8+ years. The other reason is AT&T actually advertises and attempts to sell plans to people who BYOD, and advertise advantages of taking them up on those plans. AT&T still makes money on people who have plans with them even if they BYOD, and they don’t want to miss out on that particular market. Any carrier could block a phone by IMEI, but there is not presently any indication that any carrier is planning to start blocking phones “they don’t like”, and there is no indication carriers are planning to do so as part of the 3G shutdown. There are some phones that have weird compatibility issues with SIM cards and stuff, but I think that’s very niche and happens when you’ve done something like imported some weird phone from the Chinese market to America or etc.
Since people have weighed in on it with opinion–I’ll note that all the carriers have customer-unfriendly business practices. There’s only three real carriers and some MVNOs. However not everything is malicious. A huge % of wireless customers are not tech savvy and do not understand what a 3G shutdown means. They will just experience one day having a cell phone, and the next day it no longer works. It may also be the only phone they have. AT&T with its automating distribution of new phones is likely assuming it will help far more people than it annoys, and I think that’s actually probably a fair bet. I assume the false positives like in this thread are probably niche phones (like the BlackBerry phones are) that just aren’t on some list that they’re working from, and most people whose phones are flagged as not working probably actually do have phones that will not work after the cutover. Arguably AT&T is going above and beyond what some of the other carriers are doing by trying to insure these people (many of whom may not even understand what is going on) don’t experience losing their phone service one day without knowing why, and having no option perhaps other than driving to an AT&T store to fix it (which raises its own concerns during a pandemic.)