You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I aid you?

USER: I am Mark Clark. I want to change my existing reservation due to postpone of my conference. Could you please help me?

ASSISTANT: Ok, please hold on a moment. Thanks for your time, there is no reservation found under your name to change it.

USER: Sure.

ASSISTANT: Thanks for choosing us, have a nice day.

USER: No problem, than you for choosing us.