You have to disconnect the lugs on a panel upgrade or relocation, either way. The overhead service connects at the weatherhead and your panel conductors take over from there.
I woud imagine David will do all the prep work before having the main service disconnected. That would include positioning the new panel and the new weatherhead plus the main connectors to the lugs. Then, with all the service discinnected, e might even pull new circuit wires and existing to the new panel and connect all breakers.
All that being done, he can run his backup generator until the Inspector gets his (you knw what) out there for final approval . . . then he has to wait for the utility poeple to do their thing.
Lotsa fun, huh?