That isn't true.
On another forum, my login is oloykobkob but my username is Mikasa, so it's possible to not change the login, to my understanding.
If someone is named Mikasa for example before you came and agreed to change their "display name to "bob" you couldn't take mikasa because his "login" name is mikasa so it can't override that. I just tried to. I was going to implement this at a 6 month rule, and just change the display name so that their login name would be the same if they ever came back.
I'm 95% sure it is possible, I'll look into this more, and I'll let you know the results.
Pm me if you find a way without changing their login name i've tried numerous times. thanks buddy