You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I assist you now?

USER: I am planning to change an existing reservation, due to health issue. Can you help me?

ASSISTANT: Sure, I can help you regarding your change in previous booking.

USER: Sure.

ASSISTANT: Can you please allow me to check with your previous booking for a moment? Meanwhile, may I know your good name?

USER: Yes, please. Myself John Hernandez.

ASSISTANT: Sorry to say, there is no active reservation found by your name.

USER: That's fine, thank you.

ASSISTANT: Thank you for choosing us, have a nice day.

USER: Thanks.