The Department of Education sold my loan to a private company within a year or so of graduation. No rate change or anything, I just started logging in on a different web site to pay. Same thing with my wife and with many people I know.
I wouldn't buzz in on Jeopardy with that answer, though.
If you don't believe the notice you are given, you fail to pay. If you do believe whatever notice you get, you are vulnerable to fraud.