Change Apple ID for a user when the e-mail cannot be confirmed
I spent some time with Apple Support on this but we only succeeded in bricking my system so that I couldn't create any kind of accounts (so existing accounts for Google, etc. were lost and not replaceable)!

The Apple support script suggests you delete the accounts from ~/Library/Accounts and elsewhere before running defaults delete MobileMeAccounts. Now, the acccounts in ~/LibraryAccounts are in an SQLite database which is readable and editable with the sqlite3 program so changes are always possible and removing the files should be considered a last resort. I was also required to reinstall MacOS and reset the permissions for my home directory, takes a while as it's 200 GB. I also tried reinstalling from Time Machine (takes forevever) and failed here. Fortunately, I'd also created a backup with Carbon Copy, which is both faster and more useful.

Relevant, at least for me, because I couldn't sign out from the account I can use but can't confirm, is the MobileMe stuff, which is what Apple called accounts before it introduced iCloud.

However, again deleting anything without creating a backup is asking for trouble. Fortunately, profile information can be exported and imported so you can make a backup and even make changes. To make a backup defaults export MobileMeAccounts - &gt; accounts.plist. This will create an XML file which you can edit in a suitable editor or with XCode. You can then make changes such as removing a broken account and then import it using defaults import MobileMeAccounts accounts.plist. If you then restart the machine or logout and log back in, you should be able to create associate the account with a different Apple ID. This worked for me

This still doesn't solve the underlying problem as to why Apple will not accept certain Apple ID accounts as valid accounts and, apparently, it will also not let you merge them. But they can be added as additional accounts so that purchases from the App Store, etc. should still be usable.