When do you stop calling it a preview and start treating it like the show?

I’m interested in the point where a longer work-in-progress stops getting rebuilt after every outing. Early previews feel like permission to move whole sections around, try an ending that may not survive, cut ten minutes and put five back. But at some point I assume you need a stable version long enough to learn what the actual show does rather than what tonight’s rewrite did. If you’re developing towards a Fringe run, when do you start freezing the running order and making smaller changes instead?