When we ask for a feature to allow username to change, we mean it universally. No any past name left in the site. And to reduce confusion, we can let the buyer know that we've changed our name maybe by giving notification or include it in our profile "previously (past name)". And I believe that there is a way to make it so that no one can use used username.
If you read previous comments on this topic, they are talking about limiting the maximum amount of name changing (2-3 maximum), even the mods themselves.
Well, of course we can not get rid of the fact that there might be some people who use this feature with negative intend, but I believe it will bring more goods than bads. And we can always report those who prioritize in cheating.
What I see "right" as a zero experience seller in Fiverr might be different with what I see right after some months of experience selling in Fiverr. You may be the seller that doesn't have any concerns with your name for years and I'm happy for that. But for my case, and maybe for some other sellers too, this can be a way for me to be more comfortable selling in Fiverr with an Identity I wish to be.
note: pardon my grammar since I'm not a native speaker and still is learning.