App Store Update cannot change account I am on MacOS Sierra 10.12.3. I have several updates pending in the App Store app. When I try to load them, I get asked to log in. However, the e-mail address that is pre-populated and not editable is an old one that I do not use anymore as my Apple ID. I have a new Apple ID and I am even logged in with that one on the store. 

What happened is that I setup this machine using a backup which still had the old Apple ID. For several reasons, I could not log in to the old Apple ID account anymore and created a new one. It seems my machine cannot forget about the old one. I deleted it in the accounts system preferences already.

Even when I log out of the App Store app, it still asks for the password for my old ID. Is there any cache file which I can delete to completely remove the old one?
There is no way round this.
The apps (&amp; even the OS itself) belong to the Apple ID, not the 'computer' or even the 'human' operating it. 

You would do better to try get the old account back into shape - start at https://iforgot.apple.com -  then set up Family Sharing  which would then allow both accounts access to the full purchase history.