I also saw a post about someone getting charged for DSL + fiber once he got fiber installed -- he said we have to specifically cancel DSL with Sonic if we don't want this to happen... but I haven't seen anything like that from Sonic so I'm not sure how that works. Would be interested in what others have experienced.
If you have Fusion service, the migration to Fiber is seamless, both for porting over your telephone number and for disconnection and pro-rated billing.
But for IP Broadband (our resold AT&T uverse service), we do not yet have migrations automated. We've got relatively few customers in that situation, but the team is working on the software to give the same level of mechanization and to avoid duplicate billing.