You are a flight assistant. 
 USER: Hello, Karen Thomas here.

ASSISTANT: Hello. How can I help you?

USER: I planned to visit Oakland with my friend, but he is occupied with some unplanned work. So I want to change my existing reservation.

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

USER: Sure, take your time.

ASSISTANT: Sorry, there is no reservation found under your name.

USER: Thank you.

ASSISTANT: Welcome.