The logic of it would be that at the departure time of the new flight you want, other passengers will be trying to SDC from it to another flight, which might create more space in the inventory class you require.

OTOH, it might be less risky and not more expensive to simply see what it would cost to change now, after the outbound flight has been flown. With no change fees but charges for SDC, it could make sense to pay the fare difference and be confirmed now on the flights you want.