I've recently changed apple ID and I've downloaded xCode on mac with another apple ID.
Not the mac store informs me there is a new version for xCode but I need to insert password of the older Apple ID, and I can't just use the new one. It seems the app is associated with the old apple ID.
I can't even download a new xCode using the new apple id, because it only gives me the option to update it with the old Apple ID.
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