Presently, new quizzes are given some helpful publicity (main page listing, Quizzes menu, top of “My Quizzes” list). I suggest the same helping hand be made for major upgrades to old quizzes. This would be a win-win-win situation:
Funtrivia gets an improved quality in its quiz inventory
Authors get an incentive to make upgrades (more plays, higher ratings)
Players get better quizzes to play

For example, I just added Interesting Information to all 25 questions in my old quiz “Sports Nicknames.” Among the tidbits I found was this gem:

One of the better-known teams ”Dizzy” Dean pitched for was the 1934 St. Louis Cardinals (known as the "Gashouse Gang"), who got that nickname because of their often unwashed and smelly uniforms. In those days, gas for lighting and cooking was commonly made from coal in “gas houses,” and they were noted for their foul smell.

While I love the idea of authors being able to fine tune older works there's a bit of a problem in that players will not be able to rescore, rerate, or receive points from older quizzes they've already played, revisions or not. None of the ratings, scores, or difficulties of the quiz will have gone anywhere and, in theory, the older quizzes should be down towards the bottom of the quiz list if they're already of a lesser quality.

While some authors are the type to want to improve their older submissions (and I know some editors and some real-life authors who do the same thing), some may simply find it more to their liking to write completely new quizzes to track their improvement over the years, to benefit from the bonuses you listed, and to start with a clean slate with newer players.

There is currently no system in place to refeature or wipe clean the quizzes of the past; technically, an author could go in and edit one info section, but would it count as a major revision? Authors already do this for correction notices. In this case would it fall upon the editors to designate these particular quizzes?

If it were me I'd opt to look towards the future not simply to add more to the numbers of the site (which is on my mind to an extent) but to demonstrate a visible increase in the caliber of quizzes we place online at the present time. I wouldn't stop people from working on their old quizzes for the reasons you mentioned but I wouldn't want to detract from the value and importance we place on fresh, new quizzes.

Kyleisalive, you raise a good point about affecting older scores. Perhaps I should have limited my suggestion to just adding Interesting Information and not modifying the underlying quiz/answers.

However, if you're making substantive changes (such as adding interesting info) these changes need to be reviewed by an editor.