Different validation rules for insert and update ?

I have some fields which are only supposed to be added while a new row is being inserted and while editing, these fields should not be updated.

How do I handle that in Yii ?

Also, in case a field is being inserted, it should follow a different validation rule than when the field is being updated. Is it possible to do something like this in Yii ?

Thanks and I appreciate any responses.

Understanding Scenarios