James filed paperwork to switch from No. 23 to No. 6, the league confirmed Tuesday.
The Plain Dealer of Cleveland first reported the story Monday.
James, who declined interview requests on Tuesday, is going from one legendary number to another. Julius Erving and Bill Russell both have made No. 6 famous.
"The first thing I think of is Dr. J," Cavaliers coach Mike Brown said.
The All-Star said in November that he was thinking of giving up 23 out of respect for his hero Michael Jordan, but at that time he was "50-50," according to the newspaper.
"I've done it," James told The Plain Dealer Monday. "I already sent it in. I'm going to be No. 6."
Six is James' jersey number on the Olympic team, and he wears it every day at practice, but the NBA does not have to grant his request. If James switches teams, he can choose any number he wishes. Permission for a number change is required only if he stays with the Cavaliers.
"All it does is back up LeBron's beliefs," Brown said. "He's a loyalty guy. When he believes in something, he sticks to his guns and he believes wholeheartedly in what he's doing."
He isn't the first superstar to change numbers midcareer. Among others, Jordan briefly wore No. 45 after coming out of retirement with the Bulls, and Kobe Bryant more recently switched from No. 8 to No. 24 with the Lakers.
"We remember the 8 on Kobe, but we also know the 24," Cavs point guard Mo Williams said. "After a year or two, people will look past the old number and let it rest with Jordan."
Although Bulls fans would like to see James try to make No. 6 as legendary as No. 23 in Chicago, New York also has maneuvered to have enough cap space to sign him.
Cleveland can sign James to a longer, more lucrative deal, however.
James dismissed a question about his future shortly after the Cavaliers beat the Knicks 124-93 Monday night.
"I stopped answering free-agent questions a long time ago," he said.
Information from The Associated Press was used in this report.