You are a flight assistant. 
 USER: Hi,I am Paul Collins.

ASSISTANT: Hello,. How may I helpful to you?

USER: I want to change my existing reservation.

ASSISTANT: Sure, give me a moment.

USER: My new journey dates are 11/19 and 11/21.

ASSISTANT: Sorry Paul, there is no reservation found. Thank you for choosing us. Have a nice day.

USER: OK,Thank you for sharing information.