Depends how far through trhe upgrade you went, if you've done the migration steps, i.e. applied the DSM 6.2 .pat & it has done the upgrade, then I think you'll need to follow the steps as per the OP. If it just fails to show up in Syno Assistance or something like that then yes power off, swap USB
Yes should just boot & then be visible in Synology Assistant with a status of Migratable, you can then apply the pat file for the upgrade.