The forum is hosted and uses software by Discourse.org, we didn’t create it. Discourse’s default setting allows a user to change their username up to three days after they create their account, to prevent people from changing their name frequently, which could be confusing and/or disruptive. I agree with that reasoning.
You can always request a username change from @forum_moderators, but we would really like for people here to have the same username as their iNat account, so I wouldn’t want a username to to be changed unless it’s to align it with the person’s iNat username.