Hi, There were a number of changes form Adobe Flex 4.0 to Adobe Flex 4.5 - the introduction of spark skinning for instance.
These links may help: http://www.adobe.com/support/documentation/en/flex/4/releasenotes_flex4_sdk.html http://helpx.adobe.com/flash-builder/release-note/release-notes-flex-4-5-1.html And to find out what changed since the move to Apache look at the release notes. https://github.com/apache/flex-sdk/blob/develop/RELEASE_NOTES Despite the number of changes you shouldn't need to change too much code but really the only way to find out is to give it a go as it will depend on exactly what sort of application you have and what features you are using. If you run into any issues just ask on the list. Thanks, Justin