We added that feature a few months ago, but it doesn't backpedal and pick up the old ones unless the staff forces it.
We used to allow name changes every 30 days for Premium members, but some people felt compelled to change it frequently (5-6 times) for no other reason than they could. Since changing that restriction to a far greater time span, we've had far less "indecisiveness".
Most members change their name once, if at all, and that's it. Some change, then change back, but the excessive username changes were getting to be a bit of a problem, especially if they caused trouble (although the infractions and warnings neatly follow your identity thanks to the forum software).
To be fair, I technically changed mine three times, once for a Capitalized username, and two times as a test to see who was paying attention. Turns out that you're a bit more anonymous, except for those in the know...unless you make a big announcement (which I hadn't). And then, back to where I started.