My iCloud Mail account is set up as POP on my Mac. How to change to IMAP?

My iCloud email account appears in Mail / Settings on my Mac as "POP" and will not connect to the iCloud mail server via Mac Mail. This email account works fine via my iPad Mail and via iCloud.com. But on the Mac it is stuck with some ancient setting as POP. I have tried all reasonable approaches:

1. There is no option to change the account type in Mail Settings or System settings
2. I tried disabling the @iCloud account and then adding it as a new account, but it remains POP
3. I created a TestUser account on the Mac and connected to mail, and it works; ie the @iCloud mail account connects via Mac Mail App. But I can't figure any way to transfer that to my main Mac account.
4. I have spent a LONG time on the phone with Apple support trying all of the above but without solving.

Has anyone else had this problem and solved it? I wonder if perhaps there is a configuration file which if deleted, would force MacOS to recreate this mail account properly? Any help appreciated.