In Chapter 7 of The Great Gatsby, what were the changes that Gatsby made in his lifestyle that concerned Nick and why did he make them?

This chapter opens with the news that "the lights in [Gatsby's] house failed to go on one Saturday night" and the parties at the mansion have ceased. At this point, the purpose of the parties has been fulfilled. Gatsby has re-connected with Daisy, impressed her with his achievement of wealth, and is now engaged in an affair with her that he hopes will lead to marriage. 

Gatsby's aim in throwing the parties in the first place is to do just this - to capture Daisy's attention and her love. 

The shift from huge and elaborate parties to a new, far quieter choice of lifestyle is rather drastic. Nick takes notice and asks after Gatsby, inquiring with "an unfamiliar butler" as to whether or not Gatsby is sick. When Nick finds out that Gatsby has fired all of his former servants, Nick wonders if Gatsby is going away and puts this question to him over the phone. 

During this phone call Gatsby's motives are explained. 

"Daisy comes over quite often - in the afternoons."

Nick suspects that Daisy did not approve of the parties, but Gatsby speaks to the idea of discretion in his decision to fire his staff and end the stream of Saturday night parties. Gatsby does not want people to gossip about Daisy's visits. She is, after all, a married woman with a public profile. 

With his aims nearly attained, Gatsby transitions from a very public attempt at courtship to a private affair with Daisy.