Can not Update Read-Only fields in Import from XML

Hi All Experts.
I export quote product as xml file to re-import.
I change the field "price per unit".
Price per unit is read-only and after re-importing, It does not not update.
Note again that price per unit is a look up field and read-only.
I can change the quantity in re-importing( that is not read-only field) , But can not change the price per unit.
It give me successful in re-importing data, But does not update that record.
:-?
I am wrong?