Bought it on a disk and app store will not allow me to upgrade even though it says upgrades available.
Click iMovie/Check for Updates. That should get you up to version 9.0.4. (What version are you currently on, by the way.)
If you are on Lion (OSX 10.7) or Snow Leopard (OSX 10.8), you can upgrade to more recent versions through the App Store.
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