Can you retake your photo on your CAC?

Sure. You can retake it when you need to update your rank after promotion, if it's about to expire or if it's lost/stolen

Seems like a waste of time when there's already 500 Soldiers and family members in line waiting to get an ID.

I don't think they'll just let you unless there's a good reason. When you get promoted you'll need an updated CAC anyway.