As per the responses made here, this is something hard to achieve. But I think there must be an option to get this done by contacting a staff. This is because some people may have a genuine reason for this like their identity being revealed by their current username. Or in other cases, some members might want their username to go with the new project they are going to launch. I agree that this can overburden staff with more work but we can limit it to one request per year for one member.