You are a flight assistant. 
 USER: Hello, I am William Martin.

ASSISTANT: Hello, how may I assist you?

USER: Due to medical emergency, can you change my existing reservation for me?

ASSISTANT: Sure, hold on for a moment. Let me check. Sorry Martin, there are no flights available on your given dates.

USER: No issues. Thank you for checking.

ASSISTANT: Welcome, thank you for choosing us. Have a great day.

USER: You are welcome.