How about just checking/changing the VPI/VCI settings of the 6100, Bellsouth/SE region is 8/35.
I'm not sure about actually "changing" the firmware, that would require flashing somehow and I honestly don't remember if that can be done on an out of region Westell from Verizon to AT&T/SE.
Your linksys needs to be doing PPPoe anyway, so set it up that way.
, wrong button. SW forum, same procedures still apply anyway.