If Vodafone install a new ONT to the property, nothing will get cancelled.
That wasn't the scenario described.
The OP wanted to upgrade the existing BT FTTC to FTTP.
After the upgrade was complete they wanted to use the 14 day cooling off period to migrate to Vodafone.
Vodafone wouldn't be arranging the ONT install.
No migration can be cancelled because of a fixed contract or existing debt with a previous provider anyway.
That goes back to the days of having to beg your provider for a MAC address to move away from them.
As for nothing being cancelled just because an ONT is installed.... It's somewhat irrelevant.
If you're upgraded to FTTP with the same provider you have 14 days from the order being placed to cancel that order and revert to the previous contract.
Though the likelihood of getting an appointment for an ONT installation within 14 days aren't very high.
BT could technically provide the previous contract over FTTP complicating the example given but not all providers have identical pricing for FTTC/P.