If we are wrong then things will get interesting with angry customers been throttled with nowhere to turn when they flip the switch.
Except this doesn't seem to be about getting people off copper, its getting people off POTS, by re-contracting to the same service you already have only without the POTS service.
This seems like putting the customer through a lot of stress when this should be something the ISP handles without your input necessary, other than to give you the option to politely ask them to not do so because you're vulnerable and/or have equipment that wont work over VoIP.
This seems rather unreasonable given if you DO want to migrate POTS to VoIP without ceasing your broadband, there is no consistently reliable way to do it. Openreach (possibly BTW?) are directly responsible for some people not switching due to this problem, minimum contracts, re-grade fees, etc.