Unable to duplicate requisition where Project Owner no longer valid -can this be overcome?

Summary

Hard error encountered when attempting to duplicate existing Requisition with lines attributed to Project with an obsolete owner

Content

We have a user who woud like to duplicate (and subsequently amend) a number of historical, complex Requisitions.

The user is unable to do so as several lines on the existing requisition reference a Project Owner who has since left the business.  They encounter a hard error stating that Requisition dupliaction is not possible as person X 'is not in the list of values'

We are unable to amend the historical Reqs as they have been converted to POs and consumed.