I was pleasantly surprised myself how smooth my migration to FTTP went. Though that may be partly because I didn't mind they removed my landline, despite it still having an active POTS service. I had deliberately kept my landline with BT and broadband independent to it, to avoid issues changing broadband over the years. Had I wanted to keep that line it would have been a PITA, but it made migrating to VoIP a whole lot easier in the end.
I guess after a while you just expect things to not be entirely smooth and plan accordingly.
That and when the local council run alt-net for FTTC (before Openreach put us in the plan) went bankrupt and Openreach had no procedure to migrate back to their network as database said no. That sure was fun times.
