Hi Marcia and Vicki,
There isn't a way to prevent the waiting room from showing while changing presenters. The default behavior currently is for screensharing to pause while changing presenters, which then results in the waiting room being displayed. I'll be glad to pass your feedback on to our product developers though and hopefully we can have a better solution for this in the future.
@Sean_White I'm afraid the process for changing a presenter is not one that allows for automatic screen sharing at the same time. That being said, there are some ways you can improve the presenter hand-off with this Attendee Setting (changeable online): https://blog.gotomeeting.com/simplified-presenter-swap/
AshC - That appears to be a feature for GoToMeeting rather than GoToWebinar.
A solution that allows any one of 500 attendees, on their own initiative, to immediately take control of the screen sharing and become the presenter is a scary thought.