I did think it a bit odd, as the switch is instigated once a valid registration is seen from a hub. This process has been mentioned before on this forum.
If you think about it, the phone number can only have one presentation, either on the PSTN line card in the exchange, or the Smart Hub 2 via the DV platform.
The changeover would be handled robotically, but there is a lot of things that can go wrong, especially where incoming calls are concerned, as confirmed by many post on this forum.