Apparently OldValue is the value the field held on initial load, and not updated for each time the onChange client script runs (i.e. each time you change the selection).

see https://www.servicenow.com/community/developer-forum/how-to-get-oldvalue-in-an-onchange-catalog-client-script/m-p/1761324

https://support.servicenow.com/kb?id=kb%5Farticle%5Fview&sysparm%5Farticle=KB0711972