sorry mate, but in blockchain, nothing is changed, for security reasons.

You could try to make a new account.

If the username were a pointer to a unique userid kept in the blockchain, the username could be changed while preserving the blockchain.

My question is that: if I change my username, will only the new name  appear in the next posts?

This would be interesting, because you can list your old work using this: https://steemit.com/static/search.html?q=YourOldName

But I still don't know if the content with the new name will not appear in the mentioned search.

This is the blockchain way my friend. Once done something... is forever! ;)

http://2.bp.blogspot.com/-2QB3g3qZBEY/UapJk7Gz4fI/AAAAAAAAGkc/Rzy7_3PCqsw/s400/grumpy-cat-meme+%281%29.png
# You don't.
Maybe you need to read how blockchain works. Once you put something it in you can't change or delete it. Isn't it so liberating for a social media knowing no one can delete or modify what you put on it? Also, don't lose your password, you can't​ retrieve it, no one can.

Saying you can't change your name "because blockchain" is really closed minded.  It makes no sense.  Sure the blockchain is immutable.  But then all that means is there's a record of the name change.  Don't be daft!  The feature should be enabled.  Steem is holding itself down.
I just edited this post, blockchain that!