Change of Purchase Requisition 
Use
Sometimes it is necessary to change order data relating to a purchase order that has already been created. For example, this data could involve deadlines, quantities, quantity units or the delivery address. Sometimes entire external operations or components must also be deleted.
As a result, it can be necessary to change the corresponding purchase requisition. The following options are available for this:
A workflow can be used to send all relevant changes to order data automatically to the appropriate purchasing group. This group can then display both the old and new version of the order data and take the necessary action.
Refer to
Prerequisites
The workflow must have been configured using the Customizing function individually for each order type and user.
You should have read the documentation about customer exits.