6.5 years and Apple still doesn’t allow changing the attached iCloud address
When I originally registered my iCloud address to my existing Apple ID, I had a typo. Not a big deal, just call support and maybe they can help resolve the issue. I was told that they couldn’t fix it at the time but in the mean time I should register the correct version as an alias. Googling the issue looks like a lot more people have this problem but still 6.5 years later they recommend using an alias (which you can’t use to sign in only to sent/receive from) or create a new Apple ID altogether (and lose all purchases ever made).