How to enddate Mileage Policy with existing Expense Reports

We want to change mileage policy for distance threshold from per trip to per period but there are few existing Expense Reports submitted by employees which are not audited.  However, during the audit process, if auditor rejects the expense report with old mileage i.e., per trip the employees are unable to re-submit rejected expense reports.

How can we proceed with enddating old mileage policy and create new policy which will allow the scenario explain above?

Please let me know your inputs on this issue.

Thanks and regards,

RR