The reports we have is that if you order from the fixed price menu, no substitutions whatsoever are permitted, not even substitution of side dishes. Most of those reports have been from Le Cellier. You can ask when you get to Chefs, unless your decision to eat there rides on the availability of this particular substitution.
If there is some sort of medical condition or allergy involved you might have a better shot at it but if it's just a food preference you should be prepared to be turned down.