I'm just opening up this message so everyone knows why - yes, it is easy to do in vBulletin under normal circumstances. The rule applies to everyone, even one of the mods wanted a username change and I had to say no. I'd even like to change my own username to "Ian" with hindsight.
However there are 3 reasons why:
As you noticed, someone already has the username Andrew (and as it happens, I've known the guy for 10 years! He was the 7th person to sign up) I can't just reshuffle usernames about, especially when the one you want is already taken.
The showthread pages are now cached manually (see here), so it would mean I'd need to re-cache them if a username changes (and it would slow the server to a crawl for hours doing this.
Changing usernames is very confusing for others, and if I allowed one I'd need to allow them all. One member here changed his username a few years ago when the post cache wasn't active, and then I had a flood of requests.