If an expense was in a stratus of awaiting approval when changes where made to someone's approval route, then it will keep it's old approval route.
To re route the Expense to pick up it's new settings. You will need to reject the Expense, and re submit it.
To reject this, an administrator or the incorrect approver would go to:
Click Approval screen, then select the persons approval queue it is in by clicking the magnifying glass.
Click their name and expand the Expense in question.
Click Reject.