The keyterms listed in Webmaster Tools are just a count of their occurrence on your site. We've played with this and can very obviously see our site move up and down in the rankings based on simply changing the number of times certain terms appear across our sites. We have a number of new sites where there probably aren't many other factors going into our ranking, so this is especially apparent.

We've seen these changes take on average 2-6 weeks to get noticed through webmaster tools. We've made many rapid changes to a few of our sites and it has clearly confused google. These few sites have been listing the wrong keyterms for months and we haven't seen any sign of them changing. One site even randomly switches back and forth between listing relevant keyterms and listing latin leftover from our Lorem Ipsum that was accidentally released while the site was being built.

Moral of my story is to not make too many drastic changes to your site, but if you've made a few, just relax and let google run it's nonsense. If it doesn't return to your old content, then I'm not sure what you can do.

The most important thing to learn about Google is that there really isn't much you can learn. Just pay attention to numbers and try to get a "sense" for what is going on. I'm not an SEO or Search expert, but I think experts just have a lot of time in front of the numbers.