In OM Extension, need to check PO Number uniqueness irrespective of the case on a SO

Summary:

In the Order Management Extension, need to ensure that the PO Number entered on a Sales Order is unique irrespective of the case of the letters.

For example, if a sales order already has PO Number as "POTest123", then any other PO Number like "POTEST123" or "PoTest123" or "potest123" etc. should not be allowed.

Currently what we are doing is, whatever PO Num is entered on the order, we're converting it to uppercase for uniformity (like if user enters "PoTest1", it'll become "POTEST1"). But the user does not want the letter-case to change.

So any suggestions on how to execute this??