HTC's US VP of Product, Mo Versi had some good news for owners of the One M7 on AT&T yesterday evening. After months of waiting, the Lollipop OTA is starting this very day. "Tomorrow" in the tweet below refers to today, March 31st.
AT&T HTC One (M7) Owners! Great news, we've received technical approval on Lollipop OS. OTA will start tomorrow. Thank you for your patience
— Mo Versi (@moversi) March 30, 2015
HTC originally hoped to have all its One devices updated a few months ago, but that deadline came and went before it could push Lollipop ROMs through the carrier maelstrom. The M7's Lollipop update will probably be quite similar to what you'll see on the M8 or M9, although this will likely be its last. The device is almost two years old at this point, after all.
AT&T has posted the details on the OTA, and there are two of them. The first is a 198MB update to optimize memory, then there's a larger one that updates the phone to Lollipop.