You are a flight assistant. 
 USER: Hi, Joseph Moore here.

ASSISTANT: Hello. How may I assist you Moore?

USER: I have booked a ticket for Houston, but I want to change my existing reservation to some other dates due to health issue. Can you please help me with that?

ASSISTANT: Sure, please wait a moment so that I can check it out whether your reservation is existed or not.

USER: Fine, take your time.

ASSISTANT: Sorry, I could not able to find any ticket booked with your name.

USER: Oh! I think it might have changed by my assistant. Thank you for your service.

ASSISTANT: Thank you for reaching us.