My first username was my main online tag that I started using when I was ~12 years old or so. It was really dumb, which is mainly why I changed it. I no longer use it for anything except my "spam" email address because it was stupid.
My second username, Mellow Ezlo, came about after I had been intending to change my username, but I only went by that name here (I occasionally used "Mezlo" for other online things, but never "Mellow Ezlo"). Eventually, I started using SlothyCyrus as my main online tag, which I still use for most things, but it's never been my username here. Mellow Ezlo was too special for me.
My current username is self-explanatory; it's simply my first name. I started using it here because I stopped caring about keeping it hidden, and I no longer identified with "Mezlo".
To answer the question, I don't regret my first username, but I do ask myself what on earth was I thinking. Granted, SlothyCyrus isn't much better, and the conclusion is I just suck at coming up with good usernames!