you don't necessarily need to buy up to the walk-up fare. You only need to buy up to the lowest fare class that has availability.
But, I would expect that it would have to have a corresponding fare that has a zero day advance purchase requirement.
If your ticket is in V class, and V isn't available for SDC, and X class is available - - BUT - - the lowest fare available for purchase today is B, I don't see how they can upfare your ticket to X that isn't for sale today, if the X fare requires a 14 day advance purchase.