You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, how can I help you?

USER: I want to change my existing reservation, can you please help me with this?

ASSISTANT: May I ask you for your name please.

USER: I am Helen Jackson.

ASSISTANT: Give me a moment.

USER: Sure.

ASSISTANT: We have found an active reservation by your name, so could you provide me the airport codes?

USER: My airport codes are OAK and SFO.

ASSISTANT: May I know the travel dates please?

USER: I prefer to travel on Sep 21 and Sep 23.

ASSISTANT: Is there any class preference?

USER: No.

ASSISTANT: May I know the connection limit?

USER: I prefer to travel in a connecting flight.

ASSISTANT: Is there any time concern?

USER: No.

ASSISTANT: Are there any more preferences?

USER: No.

ASSISTANT: Give me a moment.

USER: Sure.

ASSISTANT: Helen Jackson, I regret to inform that there are no flights on the given dates.

USER: Ok, no problem.

ASSISTANT: Thank you for reaching us.

USER: Thank you for your information.