You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I assist you?

USER: Donald Williams here. Due to sudden change in my planned date of trip, I would like to change my reservation. Can you please help me out with changing of my reservation?

ASSISTANT: Sure, please wait for a moment to check your details.

USER: Sure, take your time.

ASSISTANT: I have found your reservation, can you help me with your new airport codes?

USER: That's great. My new airport codes are DEN and SEA.

ASSISTANT: May I know your travelling dates?

USER: Sure, I am planning to move on 12/26 and please book my return ticket on 12/28.

ASSISTANT: Sorry, there is no flight is available for you.

USER: Oh! So sad to hear. Thank you for the information.

ASSISTANT: Thank you for choosing us, have a nice day.

USER: Welcome.