Randy Falco, president and chief operating officer at NBC Universal, has been responsible for operational functions, including network and cable sales, affiliate relations, cable distribution, network and studio operations, and worldwide television distribution, Time Warner said in a statement.
It was unclear when Miller would leave his post, why he was being replaced, or whether he was being demoted or leaving the company. "We don't know what his plans are at this point," said Time Warner spokesman Keith Cocozza.
No start date had been determined yet for Falco, and Miller remains in charge until then, Cocozza said.
"Jon has done a fantastic job of creating a strategy for AOL to succeed," Cocozza said. "The company has now chosen someone with a proven track record to be able to execute operationally."
"Randy is a first-rate choice to ensure AOL realizes its promise," Time Warner Chief Executive Dick Parsons said in the statement. "We thank Jon Miller for his four years of far-sighted leadership during a difficult time at AOL. We wish him well as he moves into the next phase of his career."
Millerat a time when ad revenue losses and a subscriber slowdown at AOL was blamed for dragging down the company's stock price.
After numerous revampings, Miller oversawthis summer from a subscription-based business model to one supported by advertisements. At the time, the company said it planned to reduce its work force by 5,000 and has been closing offices and laying off employees since.
That strategy has "shown some early results," Cocozza said, evidenced by a 46 percent rise in online ad sales at AOL in the most recent fiscal quarter.
"Rarely do you come into an organization with as many challenges as AOL faced when I arrived and then have the great satisfaction of putting it on sound footing," Miller was quoted as saying in the Time Warner statement. "I've had that opportunity at AOL over the past four years, and I'm proud of what we've accomplished. I deeply appreciate the unwavering effort and support from the wonderful people at AOL, and I wish them, Randy and Time Warner all the best in the future."