I remember reading somewhere that all old high score lists and such would have to be reset if you were to change the user names. And apparently also trophies and old messages.
To me, that's indicating that it's not programmed to have your displayed PSN id linked to some sort of "root tag" which the system calls to, but it's actually hardcorded text not linked to anything. Meaning that changing the text would mean that the program would call to something that's not defined.
Say that my name "Fridge-man" would be linked to the number "1234" in some Sony super database. So if a highscore was linked to the number "1234", the game would call for that number, and it would read out "Fridge-man". Instead, it's just instructed to write the text "Fridge-man".
But they have real name thing, so they could use a system similar to Uplay with legacy names. Make the old PSN id the "root tag" but add a maliable name ontop of that. I don't know how this would work with the highscores, if they were to be hardcoded text.
I have some very light programming experience, so I might be off by a long shot.