upgrading service

Because AT&T's system is brain-dead. 

Just curious...do you have a separate ONT that connects to the BGW320 with an Ethernet cable or does your fiber go right into the BGW320?

If the latter, you couldn't use the BGW210 even if you wanted to. 

The BGW320 is pretty similar to the BGW210 but the 320 has Wifi6 so if you have Wifi6 devices switching back would reduce the performance on them.

The ONT jack is covered, so nothing is connected to it. 

I do have a couple of things that can do Wi-Fi 6. 

Should I then just keep the 320 connected?

Yes, I did once TRY to talk to support-like talking to the wall. 

Why do they  bother with the useless chat?  It doesn't seem to understand anything. 

So yours has the fiber into the bottom SFP like the pic below.  The BGW210 won't work at all with that setup so you don't have a choice.....keep the BGW320.

Mine is exactly as shown in your picture. 

Thanks. 

So if I keep the 320 connected, will the service automatically upgrade to 500 if I do nothing?

It should but it doesn't hurt to reboot the gateway once they've told you the change has been made and you don't see it.