It is important to understand that the subcontract purchase order is a payment mechanism for the subcontract and can only be modified after the actual subcontract agreement has been modified.
If you are adding funds to a subcontract and it is incremental funding on an existing subcontract budget you only need to submit the Marked Up SPO.
If you are adding funds not previously expected, then you will need to submit the Marked Up SPO and send a budget and Statement of Work (if it is different from the original) to ocgsubcontracts@colorado.edu.