Is there an Industry agreed process for FTTP to FTTP migrations the same as FTTC (ie same day changeover, no need to contact your old provider, etc) ?
Is it different if you have Digital Voice/VOIP as opposed to number already with a third party eg Sipgate ? Is it addressed based as you no longer have a landline number attached to your property ?
For ISP's using Openreach fibre its the same as FTTC, depends on the rest so if as an example you get another ONT from an Altnet installed then your previous Openreach service will need to be ceased by yourself. The OR ONT will sit there unused ready for use in future if your return.
The Digital Voice/VOIP is even more of mess, use the transfer as a way to get your number away from the FTTP to a VOIP provider such as Sipgate so it is no longer an issue in subsequent transfers.
On Openreach it is very straightforward to move from one FTTP provider to another - they all have access to the property address and ONT details and can see the serial number etc. of the installed ONT.
It is the same Gaining Provide-led switching process as with copper services.