You can sit on until the day of departure, but I'm sure you can see how this could turn into a much bigger issue.

Re: YQ, my guess is they'll treat it like any scedule change on a YQ error ticket. You dont pay new YQ on tickets when you reroute due to a schedule change or IRROPS, but if you make any voluntary changes, your ticket will be repriced and you'll get charged the YQ simce it was never collected.