I think that if you let yourself take a DXM break and replace with Nootropics and green tea you will be able to downregulate the NMDA receptors already upregulated from the DXM use. So basically reducing the tolerance. Also what you can add to nootropics and green tea is L-Glutamine it would help too. I would also add noopept since its good for neuroplasticity; you brain's ability to stabilize/adapt for an action and after being victim of an action. The combination of all those will increase NMDA activity which will give the message to your brain to reduce the amount of NMDA Receptors and thus reducing your tolerance to NMDA antagonists.
Cycling those is a good way of not losing the effect altho I can't guarantee that there will be no change at all in your brain chemistry, unlinked to NMDA receptors.
I am only provinding my own theory of how to reset a NMDA overactivation caused by NMDA antagonists.