Flex Builder lets you change the version of the SDK that you use to compile your projects. You can select the SDK when you first create a project, or at any time you are working on a project. You can change the SDK for any type of Flex project, including library projects and ActionScript-only projects.

When you change the SDK, Flex Builder rebuilds your application and flags any code that is no longer compatible with the selected SDK.

Flex Builder includes the Flex 3 and Flex 2.0.1 SDKs. You can also add any SDK you want.

For more information about selecting different SDKs in Flex Builder, see Using multiple SDKs in Flex Builder in Using Adobe Flex Builder 3.