I think if the VIP's/donaters want to change their username, they have to send a PM to theymos. Their isn't an option to change ur username by yourself, I think that theymos has to change that in the database.
Donators and Staff members can change their display name by themselves, no need to ask theymos.
I have heard that Vip donators and staff members can change their usernames at their ease, with somekind of option for changing their display name(their login credentials are the same).
But I also heard that legendary members can change their usernames or something similar to that. So if you are a legendary member do you still have to PM theymos/BB to change your username or is there an "option" like in the case of Donators?
AFAIK Legendary doesn't have such privileges, they need to ask theymos to change their display name.