Grounded Conductor Sizing
I am assuming we are talking about copper conductors here and the service is not a parallel feed.

NEC table 250.102(C)(1) is for sizing a service with no overcurrent protection so it is sized by the incoming conductors so it would require a 1/0 copper grounding conductor.

NEC table 250.122 sizes the grounding conductor by its overcurrent protection. So if this is after a 400A breaker, your grounding conductor would be a #2 copper conductor.

I need to make this clear. You are calling for a minimum grounded conductor which is a neutral and I am assuming you mean a grounding conductor . I am giving you information on sizing a grounding conductor. If you really do mean a neutral you shouldn't downsize it from a 500 Kcmil without professional assistance since it would require a load study.

Hope this helps.