In our regular point releases of Unity, we make sure that projects from previous minor versions of the same major version are automatically upgraded when opened in the new editor for the first time. New properties are given default values, formats are converted and so on. However for major version changes such as 2.x to 3.x, we introduce several backwards-compatibility breaking changes.
While the primary visibility of this is the fact that content authored in the previous version will play back slightly differently when run in the new engine, some changes require more than a few property tweaks to play just like you want them to. These documents outlines those changes from 2.x to 3.x:
Page last updated: 2010-09-30