I'm in the same boat, with AT&T offering 5gbps for $180 (vs 1gbps for $50 from Sonic).
I'm not sure it's worth the price difference (or the hassle of switching ISP) despite my home network already being 10gbps all around. I would definitely be willing to pay the few hundred dollars of the one-time upgrade fee to get 10gbps from Sonic though, so I'm eagerly waiting for upgrades to become available. Maybe too eagerly as I've been checking these forums weekly hoping for updates on that front...
It was said upgrades would first be available to customers in areas where the CO already has XGS-PON combo cards, which was previously stated as encompassing ~4,000 customers in Oakland and SF's Cow Hollow / Lower Pacific Heights areas, and as far as I know this hasn't even started yet (if it has, it hasn't been talked about here). Since my neighborhood was one of the first ones to get fiber in San Francisco, back in 2016, I'm guessing my CO is all GPON so I'll have to wait even longer for upgrades to start happening. Realistically, we probably shouldn't expect upgrades until well into 2023 - at the earliest.
It makes sense that Sonic wants to prioritize expanding their coverage area rather than upgrading existing customers, which wouldn't bring more money. And while there's a risk of existing areas falling behind the competition and losing customers as a result, I don't think we're anywhere close to that happening as far as the average Joe is concerned. Still, I agree it would be nice to have timeline estimates on upgrades.