You sound like you know what you're talking about, could you further elaborate this? I don't care about name changes, I am just curious as to why this could break stuff
I know you weren't talking to me, but I figured I'd try to answer anyways :)
Basically, every database has a primary key. A unique identifier for that row's information. Sony built the original PSN with the username as the primary key. So any game / save file / trophy that pulls up that user's stuff from the database uses the username to find it.
Now imagine you have an old save file that looks for your username and they've gone and changed your username in the database. That save wouldn't find it, wouldn't attribute it to your account, and wouldn't sync progress / trophy unlocks.
A smart DBO would have built a generic number as the primary key (which most databases actually do by default), and left the username as a generic (but unique) field that could be updated without issue. So the game would look to that user ID number and then just pull in the username from that field in the row.