Hi!
So, I dug up an old iPhone which my daughter wants to use - mostly for show. This turns out to be a bit more problematic than planned.
Apparently, we never reset the phone when my wife stopped using it, and in the mean time she’s changed the user name of her Apple ID (so it’s still the same Apple ID but it’s username is attached to a new Email).
I can’t find any way to get this switched around so that I can confirm that it should be reset.
Any ideas? 
You’re on the right track, @tonycraine. AppleCare is probably using the same knowledge base articles as you can find yourself online. In addition to the one you mentioned, these can be helpful:
https://support.apple.com/en-us/HT204071 which linkes to this https://support.apple.com/en-us/HT203828
Good luck!
Thanks for the thorough feedback from both of you!
Since this is just to get an old phone up and running for my daughter to have to play with - I really don’t want to go around messing with my wife’s Apple ID and sign out from all her devices etc. That’s too big of a potential issue.
To be honest, I’m really surprised by how this whole Apple ID is handled by Apple for these things.
Does anyone know what would happen if I created a new Apple ID with the Email previously used? (I.e. the Email that the old phone thinks is the right Apple ID.)
Is the only unique thing the Apple ID cares about the Email/Username? Or is there an actual Email independent ID which we’re not presented with? So that it would know that it’s not the “right” Apple ID even though the Username matches?
Is “Find my iPhone” enabled?
If so, you might be able to reset the iPhone “remotely”: https://support.apple.com/kb/ph2701