App Store asks for old Apple ID when installing updates I got a MacBook that a colleague of mine had used previously. It runs OS X 10.10.2. I created a new administrator account for myself and turned off the "admin" flag on the old account.

I also entered my iCloud / Apple ID account, so that it syncs my calendars and contacts correctly, and even in the App Store, I am signed in with my correct account.

But when I try to install pending software updates (that is, OS X system updates!), the window that pops up has the the e-mail address of my colleague filled in and greyed out—and I can't change it. 



Note that I'm signed in with a different (correct) account in the App Store. When I go to Featured and click Account on the right, it has my correct e-mail address filled in:



How do I make the App Store "forget" the old Apple ID?

I found a thread where it just says "it's not possible", and that apps are glued to the original account the Mac was set up with, but that doesn't make a lot of sense, especially in a company where laptops may be handed over from one person to another.
To delete the application from the Launchpad. Hold down the icon until they all jiggle and the (x) appears. Click that on the app you want to delete, then confirm.
If you do this while the App Store's Updates page is displaying the application, you can then just update!
If not, you will have to download it again. This might mean paying for it again. But if it's family plan eligible, it will show as needing to buy it and you'll have to confirm payment, but then it will inform you that it's family plan eligible and you'll get to download it again for free.
If going through this leaves the app missing from the Launchpad (it did for me one time), then drag the app to the trash (confirming with your password if necessary), wait 10 seconds, and drag it back (possibly confirming again). That should alert Launchpad to its presence.
I hope this helps people in the future. I wasted a lot of time figuring this out, and couldn't find a clue on the web.