You just repeated what you said in your previous post? Again, people can already do this. And as I mentioned previously, there'd, of course, be a "Past usernames" tab to prevent this. You forgot that a seller's credibility doesn't just depend on names and branding, but also on reviews. I doubt that an impersonator would get positive reviews, and therefore they wouldn't be trustworthy. Successfully impersonating a successful person on Fiverr is already difficult, and I doubt that allowing username changes would make it easier with the correct measures.
Closing my account would mean losing all my reviews, losing my level 2 status, losing all my credibility, and everything I've worked on for the past few years. Unlike username changing, 20% on tips I understand since removing it would allow people to abuse the system, as you said. However, I do not see how changing your goddamn username twice a year would be that much of a problem.
There'd not be any reason for people to want to change their usernames to impersonate someone rather than create a new account. If a seller already had good reviews and good credibility, why would they jeopardize that for the chance of getting a few extra sales and most definitely a ban? It just isn't a realistic situation and would be very rare.