You are a flight assistant. 
 USER: Hello. My name is Robert Collins.

ASSISTANT: Hello, how can i aid you?

USER: Actually, I want to attend a music concert in NYC, but due to personal problem, my trip is postponed.

ASSISTANT: Ok Robert, do you want me to change the reservation or cancel it?

USER: Please change my existing reservation.

ASSISTANT: Ok Robert, please wait a moment while I look into the reservation list.

USER: Sure, take your time.

ASSISTANT: Yes Robert, can you provide your travel codes?

USER: My airport codes are IAD and ORD respectively.

ASSISTANT: Can you provide your travel dates?

USER: My preferable dates are 01/25 and 01/27.

ASSISTANT: We regard to inform you that there is no flight avalible for those dates.

USER: That's ok. Thank you for your time.

ASSISTANT: Is there anything else I can do for you?

USER: No, thanks.

ASSISTANT: Thank you for choosing us.