Time off requests are tied to a job or assignment, not a person.  If the employee requested time off for a future date then changed positions before that date, EmpCenter will warn you that the time off request is no longer valid:

Supervisor Time Off Request History tab showing warning on invalid time off request

 

If you hover your mouse over the request, you will see more information about the warning:

tool tip box when hovering over the time off reqeust with a warning

The employee will need to resubmit the time off request for approval.