If you have multiple versions of the Flex/Air SDK on your computer it can sometimes become a hassle to make sure that every SDK has all the playerglobal.swc files for all the versions that you need to compile for.
What I recommend is creating a /flex directory in your preferred location (mine is on the root of C:) where you have all your SDKs, and put a folder there called /playerglobals where you have all your versions of playerglobal that you will need.
Then delete the FlexSDK_x.x.x/frameworks/libs/player folder from every SDK and replace it with a Directory Junction (hard) link, which you can do with the following command (Windows):
mklink /J C:\flex\FlexSDK_x.x.x\frameworks\libs\player c:\flex\playerglobals
Now you have only one place to worry about your playerglobals.