When I had CenturyLink, I replaced the ONT via a JTAG cable on the new ONT. The stock CL ONT (Calix 716GE-I R2) had a 16384 connection limit, which prevented me from running high-bandwidth Tor relays. The new ONT (Calix 803G) did not.

Calix for some reason makes it easy to clone some models.

I have a post on this: https://www.neelc.org/posts/clone-calix-ont/

Now I'm in NYC with Verizon Fios where I don't need a cloned ONT. Woo! The Verizon ONT is big and has a huge power brick, presumably because of RFoG alongside GPON.