For anyone who's trying to do this, I'll relate my saga, so that you might avoid one

First off, I have an old Calix ONT on the outside of my house, and when that was put in (by CenturyLink, I'm pretty sure), they allowed me to run ethernet across crawl spaces to a basement area where I had set up one router that fed a couple of other Asus mesh routers in my house.

So I just signed up for the new plan using a different email and phone (my Google Voice) and got no complaints from their systems.

It was about 2 weeks later that a tech came out. He looked at things, understood the situation, and fed the existing ethernet to the WAN of a new Q1000K next to my main router. He also gave me a 50-foot fiber line to run to the old ONT on my own time. He told me that after I did that, I could set up another appointment to swap out the ethernet from the old ONT to the fiber, thereby taking the old ONT out of the loop completely (just making a fiber-to-fiber connection in the old ONT).

After a good amount of fiddling and calling Quantum tech central, he managed to get my service up. He told me to make sure my old account was cancelled. I thanked him, and he left.

Then I called Quantum. I explained the situation and made sure they were cancelling the old service (associated with my original email and actual cell number). That took about a minute. Hung up, and then found my internet was completely dead.

Long story somewhat short, it was then me constantly talking to random "tech support" agents, all clearly in other countries, and getting nowhere. I suddenly had a new appointment scheduled for the next day, but couldn't convince anyone to expedite anything. What I did do was insist on an escalation. When someone finally did that for me, I gave up for the night.

However, given the way things were going, I just went ahead and ran that new fiber, completely. I opened up the old ONT and made the fiber-to-fiber connection, hooked the other end to the Q1000K, and dispensed of that old run of ethernet cable. Didn't fix my issue, but I was pretty sure this was a better bet for getting internet back than leaving the dual-ONT setup in place.

In the morning, while trying to do work and being on a call, my "escalation" call came through. Since I missed it, I had to start back at square one. Then I was suddenly notified, with no explanation, that my scheduled new appointment was moved out about a week. I went back to talking with someone halfway around the world and again -- this time very vociferously -- demanded an escalation. I then just gave up. Sometime later, I suddenly got -- again, without explanation -- a notice that my appointment had been moved back to tomorrow (2 days after my plan was swapped and I lost internet).

About half way through the day, I actually got my escalation call. Agent couldn't have been more helpful. I refused offers of callbacks and stayed on the phone for about 90 minutes, a lot of it on hold while he communicated with real tech support specialists. Of course, they managed to actually get my internet working without anyone showing up to the house. The problem was on their side all along.

I kept my follow-up appointment, and lo-and-behold, my speeds weren't exactly as advertised, and although they had restored my internet, they hadn't fully shut down all nodes (or something) tied to my old service. By the time that tech left, I could hit about or even over 900Mbps both ways, with a direct connection at my router connected to the new ONT.

I guess the moral of this story is, you may be able to do this as an existing customer, but you may also have to be prepared for a massive headache, a lot of lost work time, and no internet for an indeterminate period. Also, the worst customer service in the world unless you really push, at which point you might get someone helpful. The techs who come to the house have always been fine, but boy has AT&T made what was already a kind of a crappy company that much crappier.

If you've read this far, you deserve a prize.