I think a lot of amateurs misapprehend what "locked" means, because it doesn't mean the script isn't going to change. Locking is merely a production necessity whereby the scene numbers and page numbers et al get locked down so the production doesn't have moving targets.
Once locked, Scene 16 henceforth is always Scene 16, even if you delete or add scenes in front of it. It you delete scene 15 you do so by flagging it as OMITTED but the number doesn't change. If you insert a new scene after it, it becomes 15A not 16. EDIT: This because it would be confusing to the production. Imagine Scene 16 has certain props, costumes, actors, etc., et al, required. Now imagine if Scene 16 suddenly becomes Scene 17, and imagine trying to ensure that everything planned for 16 gets shifted to 17 amongst multiple departments and half-a-hundred production documents. It would be a recipe for disaster. Easier to lock the scene #s.
Same is true of the page numbers. If Scene 12 ran from page 37 and ended halfway down page 39 after which Scene 13 starts, if you make Scene 12 longer by a page the numbering goes 37, 38, 38A, and 39, so that Scene 13 still starts in the same place. This is necessary because you only print the pages that change and to renumber the pages would force you to reprint a lot more pages just for a few changes.
Unlocking the script generally means you had some sort of collapse in preproduction where the script has been revised so heavily it's easier to do a "new white" script and renumber everything.
