I work hard to make sure projects never break with upgrades. In the past I've added features which changed the project format, but wrote in extra code to convert the old format to the new one, so nobody noticed. It's happened a few times now <img src="smileys/smiley1.gif" border="0" align="middle">
On rare occasions features have to change. I'm always reluctant to do this but sometimes it means the software is better and easier to use, and IMO this is always good in the long term so we don't all get stuck with old crap features that never get upgraded "for backwards compatibility". The only recent example is r69 changed the auto-mirror feature. This meant everyone had to tweak their projects, but it wasn't much work - only a couple of actions, and it means the feature is now more flexible too. However, like I say this is done rarely and I'm aware it's annoying if it happens too much so it's avoided if at all possible.
So I wouldn't worry, just keep an eye on the changelogs for anything that affects you. Don't forget you can always install old releases of Construct 2, and if you regularly back up your project (which you always should) even if somehow the project is corrupted you've got your backup.