If you try this, your ticket will be repriced based upon current inventory and advance purchase rules. (It might work on a return leg of a round-trip flight, assuming that there is inventory in your fare bucket on your current flight and the dummy flight you pick and that the dummy flight satisfies all fare rules for your original ticket. It won't work on the outbound, or on a one-way). Also, there's no guarantee that somebody wouldn't snatch that empty seat while you're in the process of making your flight changes.

A Premier member might be able to do this with same-day changes, but Premier members already have access to preferred seating for no charge.