I'd be interested in a clarification here: would this be switching from AT&T retail Uverse to Sonic's resold AT&T fiber service, or a true switch from AT&T fiber to Sonic owned fiber and our own service?
If the former - if you're moving from AT&T Uverse retail to Sonic's resold AT&T IP Broadband fiber, the costs are very similar (because of what they charge us for wholesale access), and there are three primary advantages: unlimited usage (because we buy in bulk), bundled unlimited VoIP, and Sonic customer service. But to be clear, it's AT&T's broadband equipment and network, we're simply reselling.
And in regards to the transition in that case, I think that each premise can only have a single AT&T service in place at a time, so they may reject the order. You could order and see if that occurs or not - but if that turns out to be the case, you'd have to disconnect from them then order from us, resulting in almost a month of downtime - yikes. I can't recommend that!
If the latter, that's easy - just order our service and we'll deliver it, then disconnect theirs. Performance should be 900Mbps+ for wired connections, WiFi will of course vary. You'll save a lot because our monthly cost is much lower, and get all the advantages of being on Sonic's owned and operated facilities. That's our end-game.