The note at the end about the new system working as a URL system for user profiles is interesting, because around the time the change was announced I saw some people theorizing that the change is rolling out because Discord wants to implement some kind of profile page. pure speculation, but it'll be interesting to see what they do when the rollout is complete.

on that note, someone I know got the pop-up to change their username about a day before I did, and they weren't able to choose my username – and when I got the pop-up, it auto-suggested my previous username. so it seems there is a pre-reservation system that does a reasonable job of letting people with already-unique usernames keep those.