Diagnosis
You cannot replace an item and change the quantity at the same time.

Procedure
If you only want to replace an old item with a new item, do not enter aquantity.
If you want to change the quantity of the existing item, make yourentries as follows:
Example
Old material/document/class: X0000
New material/document/class: X0000
Quantity: 40
The old quantity in the material/document/class is replaced by the newquantity 40.