From the help Wiki:Your username is a unique and personal account identifier in Second Life, and as such, once you choose it you cannot change it. Why not? you can change user names in every other system I’ve ever encountered? If it's a pain to do so why not charge $$ for the privilege to do so?? It’s added revenue
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