I think that the poster is asking a different question than you are answering, havasu.
If I understand correctly, the question that is being asked is this. Is the hot water from the downstairs water heater making it's way into the cold water supply lines that are serving the upstairs fixtures?
My answer would be that there are certain piping arrangments that can allow this to happen, though it is rather complicated to explain.
So if the original poster is actually experiencing a problem with this, I would say that a check valve on the inlet supply to the water heater would either fix the problem or rule that out as the source of the problem, at a relatively inexpensive price point.