As. Mentioned by other users, it is very possible to track users even if they change their display name, as the username and user ID are still the same and can be looked up, it just poses a bit of an identity issue;
Users are identified first by their display name, very rarely do we note down user IDs of different users, if such username is changed those who identified the user by it would have a hard time discovering the new account, except they know how exactly to search for it. Newbies would be exposed as they may not be too savvy on forum matters.