I see that certainly to start with the new system only applies for Openreach changes.
For broadband, when switching between different physical infrastructure I have found that installing the new connection and then ceasing the old one works fine. It allowed me to test the new connection before discontinuing the old one. I was even able to test a vpn connection via my new connection.
I have also been able to get my new phone connection to work, granted the porting of my phone number will be a sudden change.