You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How can I help you?

USER: I am Karen Walker. I want to change my existing reservation, due to some health issue. Can you please help me?

ASSISTANT: Sure, I will help you to change your reservation.

USER: Thank you.

ASSISTANT: There is no reservation found on your name to change.

USER: It's ok, thank you for checking.

ASSISTANT: Welcome.