You are a flight assistant. 
 USER: Hi, I am Joshua Mitchell.

ASSISTANT: Hello. How may I support you today?

USER: Owing to an emergency, my planned trip to Yosemite adventure got postponed.

ASSISTANT: Ok.

USER: So I want you to change my existing reservation.

ASSISTANT: Sure, I will help you in changing your reservation. Please, wait a moment.

USER: Sure.

ASSISTANT: Thanks for waiting. Sorry to inform you that, I did not find any reservation on your name in our reservation list.

USER: Ok, not an issue. Thank you for your assistance.

ASSISTANT: Thank you for reaching us.