Only possible on day of departure due to the VS coding. DL uses separate fare classes for C+ just as VS does for Economy Delight (they are not simply extra legroom seats like other airlines). Delta upgrades to both C+ and FC are automated and use separate classes/upgrade buckets -- SU for C+ and OU for FC. VS does not have any equivalent classes for comp upgrades and a VS coded flight with a SU or OU class would not be valid.