Rewrites?

Semi-finalists are disqualified from re-entry, so rewriting those is no good. However, there are a few reasons I can think of, starting with the simplest and, perhaps, best one:

1. If you don't have another story ready, and the deadline is upon you, then why not try entering a revision? Better to be in with a revision than not be in at all. Joni herself has recommended writers would do better to be in with a previous entry, even one that they haven't revised much, than to not enter--though that being said, if you only achieved an HM or below, you should probably be trying to look at your story and make some changes before putting it back in. An HM isn't likely to climb any higher without work.

2. On the other hand, if it's a Silver, it's already in the same conceptual pile as semi-finalist and finalist stories. There are some silvers that would have made finalist in other quarters, and some that are given silver rather than semi-finalist because Dave hopes to see them again. Sometimes, in late quarters, a story may only make Silver because it doesn't fit with previous quarter winners, and Dave didn't want to put it out of contention for the next volume. It's hard to know for sure, of course, but if you have a prior Silver, it's definitely worth keeping in mind to either revise it, or just to submit it into a different quarter and see if it can catch the right wave.

3. There are plenty of ways here to solicit excellent feedback, sometimes from writers who have studied this contest in depth, and know a great deal about how to be successful within it. Nobody can guarantee what will help you clear the line to Finalist, but there are people here who can definitely look at a story and help to tell you places where your story could be knocking itself out of contention.

4. Ignoring all the above, writers improve--and writers who linger here, with eyes and mind open, will certainly learn a thing or two about what can make a story successful within the Contest. There's a lot to know about being a writer, but a lot of markets have different tastes in the fiction they look for--the Contest is no different, and a lot of conversations here will help you to learn some of those. If something you learn helps you to realize hey, if I do X, Y, and Z, this story will be a much better Contest entry, then it's worth a shot, even if it got rejected straight before. It may be that what you fixed was exactly why you got that rejection.

5. Last, but not least--you could certainly take a story you liked, which didn't place high in the Contest, and then run it through the free Writers of the Future workshop process. When you come out the other side, your story will definitely be different--and whether or not it's better, it's certainly likely to be a better fit for the Contest, and in with a better chance of doing well.

DoctorJest has a number of solid thoughts on the matter. I've actually had pretty decent luck with revisions, so I'd like to add the following:

If you got an R, odds are good that something isn't working with regards to your opening. (Possibly more than just your opening, but still!) Looking into that--and, perhaps, the story's pacing in general--has good odds of leading to improvements in a story. I've revised an R into an SHM before, although I never managed to revise an SHM into anything other than another SHM. I've revised multiple R's into story sales, as well. (My Baen award winner actually got an R when I submitted it to WotF--because, surprise surprise, being massively sleep-deprived thanks to six-month-old twins does not make editing the easiest prospect. I accidentally butchered my opening, but I figured it out eventually and got some solid feedback from my writing group on how to make it better.)

If you got an HM, the story has promise, but odds are decent that it either needs some clean-up or isn't a good fit for the contest. I've sold at least one HM--again, after revising. In the particular instance I'm thinking of, the story in question had a fairly solid start but needed a better ending. (There needed to be more cost involved.)

If you got an SHM or higher, odds are good that your story is at least somewhat professional--the basic structure, at least, is probably sound. At that point, you're probably not looking to do major revisions so much as minor clean-up or a deeper dive into description or internal character observations (if you feel the need to do anything at all). I haven't personally had a ton of luck selling higher-ranked stories, in all honesty--but I've had some, and some is better than none.

Thank you @disgruntledpeony and @doctorjest I have been debating on what to do with my R stories. With thanks to community I have grown since I wrote them.