Assuming you have the issue where you are getting the error
To update this application, sign in to the account you used to
purchase it.
on the App Store app, go to Store --> Sign Out
and then do the stuff suggested here
check this fix from Beartech at https://discussions.apple.com/thread/2794280?start=15&tstart=0
The App Store used Spotlight in some way. My fix was to add the entire
HD to the "Privacy" tab in the Spotlight System prefs pane, close the
prefs and wait 30 seconds, re-open and remove your HD from the Privacy
tab. Spolight begins reindexing. Re-opened the App Store and problem
was fixed! Posting this here for others who might search later on.
Also filed a bug report with Apple.
hope this fixes the issue for you!
edit: I think this is a different issue compared to what I had. Your only option is probbaly to delete xcode, and download with your new Apple ID