When a new version of Flex is released, it usually includes changes to the APIs, compiler, and the user interface for Adobe® Flex® Builder™. As a result, if you were working on a project but then upgraded your version of the SDK, you might have to refactor some part of your code to be compatible with the new version. In addition, if the audience for your application is restricted in what Flash Player it can use, you might have to make certain concessions to ensure that your application runs without errors.

Flex Builder 3 lets you choose different SDKs, based on your needs. This lets you maintain projects in Flex Builder 3 that have not been updated to be compatible with the latest version of the SDK, or lets you use Flex Builder 3 features without having to refactor your Flex 2.0.1 projects. For more information, see Using multiple SDKs.

The differences between Flex 2.0.1 and Flex 3 go beyond new features. The layout schemes and the styles of many components have changed. If you want to use Flex 3 features but have your application look and feel the same as Flex 2.0.1, you can specify the version whose styles you want to use with the compatibility-version compiler option. For more information, see Backward compatibility.

With current Flex tools, you can target your application toward a specific version of Flash Player. To do this, you use the target-player compiler option. For more information, see Targeting Flash Player versions.