James Gatz becomes Jay Gatsby at the tender age of seventeen when he first meets the super-rich copper magnate Dan Cody. James takes the opportunity of meeting Dan to reinvent himself and to metamorphose from a humble Midwestern kid to an urbane, successful young man.
James has been waiting for this moment for an awfully long time. For many years he's been yearning to escape his humble background and become rich and successful. In the figure of Dan Cody, he sees a role model, someone to be emulated in his search for wealth and social respectability.
Of course, a change of name is not going to be enough to give Jay, as he now insists on calling himself, everything that he wants. But at the very least it will allow him to start thinking like a rich man, which is an important prerequisite to actually being one. James Gatz was a nobody, but Jay Gatsby is someone else entirely. Though residing in the same body as James, Jay has a completely different soul, one controlled—and dare one say, corrupted?—by the overwhelming desire to be rich and respected.
James may not know it yet, but in changing his name, he's taken the first step on a path that will eventually lead to wealth, success, and his premature death.