You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello. How can I help you at this moment?

USER: Could you please change my existing reservation due to postpone of my trip?

ASSISTANT: Sure, I will help you with that. Can you please mention your name?

USER: My name is John Clark.

ASSISTANT: Ok, please hold on a moment.

USER: Sure.

ASSISTANT: There is no previous reservation available under your name.

USER: Not a problem, thank you for checking.

ASSISTANT: Thank you for reaching us.