Linden Lab very stupidly did use the exact text of your username as the indexed key for almost everything associated with your avatar. As a result, doing a rename requires tracking down every asset that you ever were associated with, anywere on the grid, and changing that association. A competent programmer would have associated the name itself with an unseen unique key value, like the UUID for an avatar that is used in their more recent code. But they made the mistake long ago, and it's fester