I agree that authors should be able to make an immediate edit to change the license -- you post, only then notice the default license, and can't fix it. That's frustrating. The workaround of deleting and reposting is extra hassle -- and, for answers and for people who can see deleted posts, adds clutter. It would be better for authors to be able to fix that. What's a reasonable grace period for this sort of edit?
That's about immediate edits. The broader use case (implied by your title) is interesting too and requires more thought. If you're the only person who has edited the post so far, it does seem reasonable that you should be able to change the license -- nobody else is invested in it yet. The implementation could also get complex. Some questions:
What should we do about tag-only edits? Tags are metadata not content, so maybe they shouldn't count against the author's ability to change the license.
But if we say that, then are we opening the door for trivial edits -- for an author saying "look, somebody corrected my one-character typo; why should that block me?"? Which might then lead to a need for editor approvals of license changes, and suddenly this is getting too complicated for its benefit.
Does the license status of a question affect answers and answerers? If the license for a question is changed, is there a reason for people who answered to object? (Real question, not rhetorical -- I have no idea.)
If we only supported immediate license edits ("immediate" to be defined), would that address enough of the need to be worthwhile? Or would that not be helpful without the broader approach?