Lets look at this issue from another perspective (my perspective). Same issue, but unsolvable even if log ins could be changed. I had an employee working for me who is no longer an employee. I had bought him a new Macbook Pro, then 5 months later he was gone. It was a better Macbook Pro than mine, so I backed up my pro to time machine, erased everything on his machine (he is not a user anymore for the machine, just me) and set up the new macbook pro using my time machine. Like I said, there is no record of this past user in accounts, the cloud, iTunes, the App Store, only me now. When I go to update an app, though, if it existed on the machine before I took it over, I can't update it. I obviously don't have his password and I can't switch the log in screen to my username because this is greyed out. What this means, then, is I can't update things like Garage Band or iMovie, or email, or anything which come with the laptop. It also means that i can't ever use the convenient "Update All".
Surely there has to be a way around this issue other than deleting these apps and then buying them new.