I would certainly have thought that a clearance such as "descend FL120" without any stated restriction would over-ride any previous clearance to FL130.
Why not make the clearance (in the specific instance quoted), "afer passing GWC descend FL120"? This would clearly show that the "new" clearance was specific to a point beyond the present clearance and that the present clearance (130 at GWC) still applies. It would also get around the "new" replaces the "old" argument, wouldn't it?