The Dispatch Release is a planning tool. One in flight is the PIC's call subject to the usual monday morning quarterbacking. I doubt the FAA has a dog in the fight unless the alternate used is not an approved airport as listed in the Ops Specs. Even then it may not be a violation if emergency authority is used.

That being said, there is no way I would want to go off the reaervation with a plane load of passengers and crew.