This page has the purpose to provide further information about the standard Behavior with Unit of Measure Changes (UOM) in Purchase Orders created from Shopping Carts.
Change the Unit of Measure from a Purchase Order created from a Shopping Cart is not allowed by standard. This page provides further information about this standard behavior and a possible workaround.
These are some points which you need to know:
a) You cannot change the UoM in the PO from what it was in the shopping cart.
b) Some Dimension could be not valid, in order to know which one is valid please go to table T006 and enter in field DIMID the dimension used (Example AAAADL), and you will see UOM that have the same dimension and then can be used.
c) For a material which is having alternate unit of measure (other unit of measures) maintained in a contract and same contract is referred in a Purchase order item, system will not allow the change of UOM. This is standard behavior, the UOM which is maintained in for the material in contract will only be considered for the PO item as well. It is important to update the contract details which expect the PO item to report the same UOM.
After applying SAP Note 1152021, you would be able to change this behavior, however it could cause some issues when posting Goods Receipts and Invoice afterwards. If you are using Contracts which has UOM defined to items, even applying the note will not be possible change the behavior.
SAP Note: 1152021 Changing unit of measure in purchase orders