Exactly. If you already have an active AT&T LTE phone, you can simply drop the SIM into the Nexus 5 and it works fine. But it seems that AT&T is balking at adding/changing to LTE data on an existing account that isn't already configured for LTE. Until somebody either gets AT&T to do it via the phone or some corporate type employee OR snagging a display device's IMEI number and having that work, it's an open question today.
Ugh. That's the boat I'm in. Came from the Galaxy Nexus, now do not have LTE. I guess I can try taking it into a store, but, I feel I won't get far.
*EDIT: What if I change to a newer data plan? Or will they really not enable it just based on the "unknown" device?