I am a Technical Lead on the Downstream solutions. Does your question specifically concern the Procure-2-Pay solution ?
If so, one of the options you have is to create a Changed Order, which will create a V2 of the Requisition and once it's approved a V2 of the Purchase Order. In the newly created Requisition (V2) you will be able to change the Unit of Measure of the Item you ordered.
The Receipt will then be based on the new Unit of Measure that was chosen.
To be able to change the Order, some parameters have to be turned on by the Ariba customer support. You can contact them if you feel this solution answers you need.
Hope this helps.
Would that potentially send a new PO or change order to the supplier?
I'm not sure I understand the question. In fact, in P2P, whenever you click on the "Change Order" button, a new version of the PR will be created and when approved, a new version of the PO will also be created and the supplier on the AN will see the old PO with status "Obsolete" and the new one with status "Changed".
Please note that not all changes that you make in a PO will generate a new PO on the AN. In fact, some changes are considered internal (example: Accounting information, or Title)
Hope that answers your question.