Will SOGEA still be available after FTTP becomes available in an area? If so, how long is it likely to remain available for existing SOGEA users?
Yes, and it will remain available until a copper "stop sell" applies in that area. At that point, if FTTP is available at your property then you'll no longer be able to order a new SOGEA line, regrade your existing one, or migrate it to a different ISP.
However, you can continue with your existing line under your existing ISP: it won't be pulled away from you.
Openreach are counting on natural churn. *Eventually* I imagine Openreach will apply pressure to your ISP to get you to migrate, but I've not yet seen any scheme to forcibly terminate SOGEA service, because there's no hard cutoff time for copper withdrawal.
(That's unlike the PSTN withdrawal, where there's a hard cutoff of Dec 2025. Plans have been announced to start intermittently degrading broadband service on WLR lines, to try to force users to contact their ISP and get a regrade to SOGEA. Whether people will act remains to be seen)
However if you are with an ISP within the BT group (BT, EE, Plusnet) then they are likely to be proactive in moving you over to FTTP when it becomes available, and you may have to fight if you want to stay on SOGEA.