You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I assist you for the day?

USER: Can you help me to change my existing reservation?

ASSISTANT: Sure, I am gleeful to help you with that changing reservation. May I know your name please?

USER: Thank you. I am Donald Lee.

ASSISTANT: Please wait for a while, let me check your previous reservation status.

USER: Sure.

ASSISTANT: Thank you for waiting, but I am sorry to say that, there is no active reservation found with your name in our database to change it.

USER: No problem, thank you for the checking.

ASSISTANT: Thank you for reaching us, have a great day.