While this seems like a good idea, I think there would be a few things to take into consideration too.
There's more likely than not going to be players who change their usernames many(many, many) times, which might add up to become an endless list of usernames they had previously been, which could clutter things up a bit.
Maybe this feature would be a temporary thing on user's accounts? Such as, maybe after a month or so, the "Formerly ____" line disappears. Or, to make things easier, it only shows the most recent few users they had before the current one?
All in all, I think this would be a helpful little gem to add!