It's not really an "upgrade" so much as just an exchange for a higher level PS award by paying mileage difference between your coach award and a PS award with VS miles by going through VS. They won't give you any kind of break in mileage requirements just because you already have seats booked on the flight. It would be dependent on their being availability in the Delta Premium Select cabin for redemption with VS award miles. If there is no award availability in the DL PS cabin for redemptions with VS miles, you will not be able to exchange for a PS award. DL may make an offer for an online seat upgrade on Delta website with DL Skymiles (not VS miles) or cash. But there's no way of predicting what the offer will be or if it will be offered or not.

Basically, you should mostly plan on sitting in the cabin you booked when redeeming VS miles on DL flights and if you want to be in a higher class, you will need to find DL flights that have availability for award redemptions with VS miles in those classes and book those flights instead.