We're very careful with changes and give thorough deprecation warnings. When you upgrade and the meaning of something that you're using is going to change in the following version, you will get a warning to change it now, with specific detail of the new spelling of that feature. So as long as you don't skip major versions, you're safe. Corollary: don't skip major versions. Even if you're upgrading from 0.4 to 0.7, you'll want to upgrade /through/ versions 0.5 and 0.6 instead of skipping them.

Aside: implementing deprecations is often harder than implementing new feature themselves. We put in the effort because it is absolutely critical that people's code never just silently change behavior. Syntax and variable scope deprecations are particularly hard to implement, but all of the changes in 0.7 do have deprecation warnings – some of them with absolutely crazy implementations that we're very excited to get to delete when we release 1.0.