Why do I need to wait to change my username again, each time I change it?
Every time you change your username, the waiting time until you can change again increases. We warn you about that every time you are about to change it.

Unfortunately, if we allowed users to change their username frequently, they would use this benefit to attack users and do other things that would be against our terms of service.

Usernames are not meant to be changed frequently, and this is why users have to wait every time they change it.
Other big social networks also don't allow users to change their name frequently (or at all).

Changing username will keep your previous username reserved for a while (meaning that no one else can use it straight away), and the previous username will redirect to your new username.
The reason we do this is to prevent confusion to users who knew you with the previous username, and to prevent abuse of the system.

Changing username on Similar Worlds shouldn't be used as a way to protect your privacy.
All your posts, comments, and any other activity would continue to be linked to your profile, and show your new username.