I'm looking to understand exactly what happens at the exchange to move a FTTC service from one ISP to another.
I wouldn't know exact figures, but going from day-to-day experience, I would say the vast majority of customers moving their broadband service from one provider to another, will also move their PSTN (Telephone service) at the same time.
It is very rare I see 'Shared' (SMPF) circuits these days.
With that in mind, there's every chance that PSTN work will be required in the Exchange, but as mooted above .... the actual broadband part of the circuit will not require any intervention at the Exchange.