This happens ALL the time. American will cancel the AM flight if it is not full and put everyone on the PM one. A few weeks ago when returning from SF (taking the red eye to Miami) we got to enjoy a nice 7-8 hour layover because of this.

Even MORE of a pain perhaps is when we have guest leaving here to depart on the first AA flight from Belize, and they take Tropic/Maya to the International only to find out it's cancelled. At least Jet makes a decent bloody mary.