IIRC, there's something of a community standard concerning this. In short, if you change your name, you incorporate a bit of the name you're commonly known as into the new name, at least so you can pick out a similarity reading the name out loud.
Example:
Toast
Teist
The prime difference is an "ay" instead of an "oh" sound, so It's close enough. Alternatively, avatars and signatures are used as the primary mode of self representation and identification rather than usernames. It's common for users to change names and keep the same avatar/signature. So long as this agreeably loose standard is followed, we don't really have any issues with user identification.
PS: Not mentioned: Anagrams. Tasty Sauteed Onion, etc.
