Hello BradP,
Of course, there is always the obligatory "cool uri's don't change" preface, but that's too much of a blanket statement.. lets find out a couple things.
Are you in a small enough niche to compete for the short tail?
I think your on the right track with a 301 redirect, and you may also want to look at the canonical tag as well.
As far as trying different things on a single post/page, it's kind of hard to keep track of things like that on a production website, because it could take months before any change is recognized and reflected in the SERPS.
Here was my discussion, and it contains some useful links that still apply:
[webmasterworld.com...]

I don't know if I can rank for the short tail or not, but of course that is my hope. Currently, some posts targeting the short tail rank at around place 70 or more in Google. That is not good, but I only have one back-link from a major site. I can write about a dozen or so guest posts for large sites, if need be. When looking at some other search engine results, it seems like that would be enough to get me on the second page for the short-tails I wish to target. At an estimated 1 percent click-through rate, that would way outperform the long tails for which I rank on Google's first page.

But the main thing I am worry about is whether changing the URL would affect SEO just based on the act of changing it in its own. For example, if I am targeting "soccer moves" and I change the url from "/good-soccer-moves" to "/great-soccer-moves", that should not cause a significant difference in ranking, assuming I properly implement a 301. Right?

If I were in yout place, I would try to keep the existing pages on the site, and add new pages for trying to target the more competitive shorter terms. It would require more work, but in my opinion would give the best chance of success in the long run

So you are saying not to use a 301 redirect? I do not want to change the pages; I just want to change the headings and titles.

Well, it's not clear to me that changing the URLs would necessarily help improve rankings very much, if at all. Some experts even think that if a page's URL, title, and header are too similar, Google might impose an over-optimization penalty.

So from what I understand about your situation, I would recommend that you keep the existing pages (URLs), but change the titles and headers to target the short-tail terms.

Another thing to consider is that Search engines like Google might downgrade the rankings of a site that has an excessive number of 301s.

Hm, okay. I appreciate your input. I would probably have to change about 60-70 urls, so I guess your advice is quite pertinent. However, on the new pages I am creating, I am definitely not getting any penalty for too much similarity between urls / headings / titles. They are all descriptive of the content. The only reason I originally used different ones is because I tried to create entertaining headings for my readers.