We corrected the "Original Hire Date" and "Adjusted Service Date" for all employees by manually "rehiring" each employee using the "Manage Employee Information" function.
We selected "Hire Employee", then chose "Edit [employee name]".
At the "Employment" page, we entered the new date and answered "Yes" in the "Rehire" drop-down.
Once we completed the process, the correct date was applied to the employee record.