I couldn't update iMovie today at the lab where I work. It just said:
"Update unavailable with this Apple ID
This update is for an app downloaded with a different Apple ID. Sign in with that Apple ID and try again."
I tried deleting the half-downloaded app from the Applications folder, restarting the computer, and that fixed it.
But then it happened a few more times on other computers, and by the third time my fix didn't work anymore!
After trying a few things, including signing out and back into the app store, restarting everything, re-indexing my whole hard drive on spotlight, deleting iMovie and copying a version that worked on another computer into the failing computer's Applications folder, here's what worked:
When I started the App Store update, it made another copy of iMovie in the Applications folder (in addition to the one I copied from another computer), with the same name. Bizarre, but whatever.
I noticed that Launchpad had a "downloading" strip in its icon. Clicking that in the dock brought up a page of apps, one of which was a blank square with "downloading" written underneath. So I clicked and held on it until all the apps started wiggling (with little x's, iphone-style).
I deleted the delinquent app, then tried the app store again. Now it didn't even show any need to update - everything works fine.