Username system changes are a flawed idea and here's why

Accounts will be stolen/phished more frequently. Considering that there is still no properly secure token storage on PC/Mac, tokens are extremely easy to steal by any app that is given access to any level of storage. Two factor auth won't help you when your token is stolen.

You will be putting an insane amount of pressure on your support team because of people trying to get back stolen accounts. The fact that there are no longer discriminators will cause a boom in the economy of people cracking accounts to sell them for profit. This economy already exists but in a much smaller capacity. Microsoft themselves made the change to add discriminators to already existing usernames to keep people happy and attempt to quell the stolen account economy.

All of this is probably known though, the support article on the change is lazy and self-contradicting. Linking to a 5 day old reddit post of someone who knew the concept of an discriminators, just not what it was called. Half of the article is spent complaining about how discriminators are confusing because they are case sensitive and allow unrestricted unicode, which were just awful intentional choices from the start.

Keep discriminators, limit the characters used in the current usernames, and then allow people to have global display names, it's really not that hard!