You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: I am Karen Lewis.

ASSISTANT: How can I aid you?

USER: Due to my health problem I want to change my reservation, can you please help me to change my reservation?

ASSISTANT: Sure, please wait for a while.

USER: Thank you, I will wait for your reply.

ASSISTANT: I found an active reservation under your name, may I know airport codes?

USER: My airport codes are OAK to DFW.

ASSISTANT: Please provide me your travel dates.

USER: I would like to travel on 06/02 and return back on 06/04.

ASSISTANT: Sorry Lewis, there are no flights available for your desired dates.

USER: That's fine, thank you for your service.

ASSISTANT: Thank you for reaching us.