Processing constraint issue

Hi All,
We created the processing constraint for one of the attribute as below....

Operation: Update
Attribute: Ship To
User Action: Require reason and require history
System Changes: Never after Insert
User Changes: Never after Insert

Conditions
Scope: Any
Validation Entity: Order Line
Record Set: Line
Validation Template: Booked

So based on above configuration when ever you insert a new line on Booked sales order also its asking us to key in the required reason.
But the requirement is.... 'Required reason' is required only if any one update on the existing line not on the new line.

Thanks,
Dhasa