Yes, you're right, upon looking at it, yahoo does appear to have changed some posters' screen names over the past few days, while not changing others. It is unfortunate that Yahoo doesn't notify people of these things.
What you are seeing is the change yahoo put in place, not posters. Apparently they did this today. I did nothing to have this change take place. Nor did anyone else. So you'd be wrong. My profile hasn't changed since I signed up five years ago.