You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello. How can I assist you?

USER: I want to change my reservation, can you help me?

ASSISTANT: May I know your name?

USER: I am John Hall.

ASSISTANT: May I know your new travelling dates?

USER: My trip dates are 12/20 and 12/22.

ASSISTANT: Can I know your travelling airport codes?

USER: My airport codes are from OAK to DFW.

ASSISTANT: Do you have any other preference?

USER: I want to return in the evening.

ASSISTANT: Sorry, no flights available on your route to alter your ticket.

USER: No problem, thanks for checking.

ASSISTANT: Thank you for reaching us.