Is there any restriction for changing username on btt?
Yes.
It is not possible (to do yourself), unless you have a really good reason to do so, and i'd presume a high level account ( Sr/Hero/leg). If that's the case you should email either theymos or Cyrus, and they *might* be able to change it for you in a few months.
Or, you can donate 10 bitcoin as OmegaStarScream already stated.
https://bitcointalk.org/index.php?topic=285427.0Atleast that's what a mod said a couple years ago.