This is just silly:
If I try accepting (no, I wouldn't go through with it), it just says that the update is unavailable. So why does it ask me to downgrade, and how do I get rid of this?
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The App Store can get confused for several reasons. I find it silly as well when it's confused but untangling things is a bit prickly and removes the humor in most cases :-(
The first and most likely is that somewhere on your filesystem an older version of the Yosemite installer exists and the store is seeing an "update" to offer. It's not really designed for the case where a downloaded app then installs the OS and clearly doesn't contemplate that the installer you "update" will in fact not run since downgrades do not happen by design on OS X.
Here are some of the various situations where clearing caches might help as well if you can't find the app itself and compress/delete/move it to a folder where Spotlight does not index it:
My personal favorite is to edit the App itself to remove the _MASreceipt files that indicate an app came from the Mac App Store itself:
This has been driving me nuts for 4 weeks now! I found a copy of the original beta install file in an archive folder of software I've downloaded. After deleting it and emptying trash, then running the following command it disappeared.
sudo /System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user