You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I help you?

USER: My name is Michael Lee. Due to sudden change in my plan I want to change my existing reservation.

ASSISTANT: OK, please give me a minute so that I can check with your details.

USER: OK.

ASSISTANT: Sorry, there is no existing reservation found.

USER: It's OK. Thank you for your quick response.

ASSISTANT: Thank you for choosing us. Have a good day.