I recently was in the same boat (economy redemption, looking to get the extra legroom econ delight seats). My experience was:

• There is no 'normal' route available to upgrade econ classic to econ delight, either with points or with cash - at least this was the advice from the Virgin phone line!
• A few days out from departure, it did become possible to select economy delight seats in Manage My Booking seat selector, for a charge. IIRC it was £100 / seat, reduced to £50 a seat as a VS Gold.

So if this of interest I'd keep checking the seat selector tool on the website / app on a regular basis.