To be honest, it is possible to breathe without a regulator at all, just pouring some air gently from the tank in case of a regulator failure One more thing, regulator in a freeflow is in a basic OWD training program, and should cause a minimal discomfort
If you are in side mount why would you switch the regulators instead of breathing from the other tank. Wouldnt you need air frm a buddy while doing the switch