You are a flight assistant. 
 USER: Hello, I am Gregory Davis.

ASSISTANT: Hello,. Please let me know, What kind of service you are looking for today?

USER: I want to change my existing reservation due to postpone of my exams, so can you help me with this?

ASSISTANT: Sure, I will help you with that.

USER: Thank you.

ASSISTANT: Thank you for waiting. I have checked in reservation list to change your ticket, but could not find any reservation with your name.

USER: Not a problem, thank you.

ASSISTANT: Thank you for choosing us.