AT&T DSL → Re: Upgraded to 2wire modem all webpages redirect to ATT

I think you are right about being wrong. I encountered the same thing when I tried tp bridge my SpeedStream 4100 a couple of years back. What I failed to do was to stop the PPPoE session completely. You probably will have to first change to "Connect on demand", then disconnect from the device GUI; you want the PPPoE session stopped cleanly before making the hardware changes. If the DSLAM thinks the session is still active during the hardware swap, it seems to balk at a new connection attempt.

If you go the 'attreg@att.net/attreg' route, you will probably lose your existing email, and encounter billing errors.

Well funny thing, today just happened to swap out my out Netgear for a new D-Link router in bridged mode..
Did not terminate session. just unplugged and set up the Dlink. Inputted the PPoE info and off and runningg. It even allowed set up by WiFi which older routers didn't used to

The thing is, I did not swap out a router. I started with the SS4100 in "PPP is on the modem" mode and the router, a D-Link D-655, set to "Dynamic IP (DHCP)" mode. Based on information that, in NAT mode, the SS4100 might not be passing Protocol 41, needed for IPv6 tunneling, I decided to bridge the SS4100. I just did the changes from the respective device GUIs, without regard for the state of the PPPoE session; and got the walled garden with the suggestion to reset the password.

I suspect that a PPPoE session which isn't fully stopped confuses the PPP server?

Isuppose that could be. Never used the 2 Wires I have one of the old Westels and it been bridged so long don't remember any issues if there were any but don't think there were.
I would think bridging it though would kill any existing PPPoE session. On the Westell the INTERNET light is out (indicating PPPoE connection) and inactive when bridged.

Interesting thing though, I gave the new router the same SSID and PW, yet all my devices saw it as different and had to re-log them all in. I guess maybe the MAC address is tracked too for auto logins

SS4100 isn't 2Wire, but Siemens (nee Efficient). Going from PPPoE to bridge caused the login server to barf when the next log in was from a different device (the router).