Users can change their usernames by requesting assistance from a moderator or someone with database access, such as Greg Bernhardt. The process involves specifying the desired new username, and if it is already taken, alternatives can be suggested. For example, if "jt" is unavailable, "jt128" could be used instead. It's important to check the availability of the desired username before making a request. Username changes are manageable with the right support from forum administrators.