You may only change a reference, if a program sponsor has not contacted such reference and/or a reference has not responded to your request.
See Also:
Who does AmeriCorps consider a valid reference?
How many references do I need?
How many references can I enter?
What happens if I have more than two references?
How do I know if my reference responded?
If there is no response from a selected reference can I resend the reference request?