DO. NOT. NO-SHOW. If you do, you are immediately labeled the worst kind of person and your Chapter Leader will probably hate you.
If your plans change and you can't make it anymore, take the 30 seconds it requires to come back to the event page and UNREGISTER from the event. Then be an extra nice boy or girl and shoot your Chapter Leader a quick message letting them know you can't make it.
How to unregister should be pretty obvious...
As should how to send a quick message to your Chapter Leader