Once you register with a username, and you get the username change via shop or other methods there's a delay on how long that previous username is stuck in the void before being used again.

It was previously 365 days
It's currently 60 days

Meaning if someone registers as Shy and changes it to Kai, in 60 days from the name change, the username Shy will be up for grabs again.

Answer solved, Closing Thread.