My campaigns seem to have a tendency to undergo fairly significant changes in between first concept and final execution. Not all, certainly. My first campaign, with pre-campaign prep of less than two weeks, stayed more or less the same all the way through. The game I ran last summer was similarly constant, but since the whole of the concept was "run Keep on the Shadowfell" that wasn't really surprising.
But then there was Is This Fair, which was originally going to be elementally themed fantasy superheroes. Then I got Stormwrack (and a player on a pirate kick) and jettisoned the earlier idea in favor of island hopping adventure. By the time it actually hit the table I was running a murder mystery, and the campaign as a whole ended up as a loose retelling of the main plot of Oblivion and a vehicle for Captain Blank's antics.
The Desert Game got it even worse. It didn't go through quite as many iterations, and not much changed during the (quite short) planning phase of the game. But once I got to it, I decided that the "adventurers academy" idea I'd had just wasn't interesting, and the PCs soon found themselves teleported (in spectacular, climactic battle fashion) to a mysterious desert land. Which they wandered around in a bit, until we all got bored with it and its fancy names for dwarves and elves and werewolves and they settled down to lunch time monster hacking in the evil sorcerer's tower.
And that only covers the games that actually saw the light of play. I've got a lot more campaign ideas rattling around that I worked on for a week or two, then dropped in favor of something shinier, since I didn't have the pressure of immediate play to get me really working. I do try to recycle that material, in some form or another, so most of the more interesting one should see some use someday.
This came up now because the same thing is happening with the game I've got planned for next semester. I've been working on a weird science tropical island hex crawl thing for about a month now, and I'm kind of starting to lose interest in it. Or rather, I've stumbled upon some newer ideas and am starting to get more excited about them.
At first I was sort of frustrated, especially since I've always kind of admired those guys spend ages building wacky detailed settings. But now I'm thinking it's just kind of my natural game-planning process, when I'm given any length of time to do it in. It's not like I'm under any kind of time crunch, since I know from experience that I'm fully capable of running a decent game with just a sketch of a setting. I'll hang on to the really cool bits (Gamma Knights!) and I may well end up back working on my little patch of hex terrain once I've worked the other ideas out of my system, but I'm allowed to put it aside for a while and play around with something else.