If I edit the webinar before it starts will it change the URL or prevent anyone from joining the event?

No, editing the webinar - or any of its' options - will not cause or prevent anyone from joining the event that have already signed up.

The only exception to this is when you cancel an event.

You can cancel an event by:

1. Log in to your Account Dashboard.
2. Click on 'All Events'.
3. Click on the event that you wish to cancel.
4. Under the 'General' submenu, click the 'Cancel event and prevent further registration' check box.
5. Click 'Save'.

Your event will now be cancelled.