The first attempt was not following a guide. It was by going into the AT&T Software Update settings applet (my phone is on b140 and b150 was downloaded this way). That would start to load the update, reboot the phone into TWRP and then sit there. Subsequent booting into the OS would only re-attempt to install the update which would again reboot to TWRP.
After recovering my phone from a pre-update backup I tried, somewhat, following this method:
https://forum.xda-developers.com/android/development/guide-how-to-update-b130-to-b140-b150-t3555254
I was able to install the stock b140 image from my SD card but where it said TWRP would be gone, mine still had it. Attempting the secret menu would start the firmware update which would, again, boot to TWRP and sit there.