Oh, I see, you mean to change the LFS username.
Ok, suppose it's possible, what happens to all the records in LFSworld and server statistics, will they be automatically transferred by use of magic to the new username? Any solution here would result in such a mess.
I think a much better option would be the ability to "unlink" a certain LFS license (S1, S2, or S3) and transfer it to a newly created username. That way, you will anyway have to start from scratch with the stats, but at least you don't have to pay for a new license.