Why does Gregor change his mind about having the furniture removed from his room so that he can move freely about?

He realizes that the furniture does not help him; he does not have the need to live like a human because that is not who he is now. He needs to be able to move; he assumes, we think, that if he can move more freely, his life might be better.

The furniture provides hiding for Gregor, a place to escape within his own refuge.