Will Flash Builder 4.5 work with existing Flex 3 or Flex 4 applications? Can I use the new features in Flash Builder 4.5 with earlier versions of Flex?
Yes, you can take advantage of the majority of the great new features in Flash Builder 4.5 when writing and maintaining applications that were built using Flex 3 or Flex 4. As you begin to use the Spark component model, you can continue to use the MX components from previous generations of the Flex framework. Using Flash Builder 4.5, you will have the option to build applications using the Flex 3, 4, or 4.5 version of the framework.