If you are worried about the changes introduced by AS3 and would like a cheatsheet for a quick reference of most of them, then you should take a look at http://actionscriptcheatsheet.com/
This site provides some interesting printable quick reference cards for the ActionScript language. AS3, Flex, AIR, Papervision, and more.
Unfortunately the site seems to be inactive for almost a year, so I suggest you to download them quickly.
This is the ActionScript 2.0 to 3.0 migration cheatsheet
In the downloads page you will find more.
Hope this will help you in the migration.