Ryan Hunter-Reay will replace Jeff Simmons at Rahal Letterman Racing for the rest of this season.
The American will make his IndyCar Series debut at Mid-Ohio this weekend, where he finished third in Champ Car back in 2003.
"I am very happy to be joining a quality organisation," Hunter-Reay said. "This is a strong opportunity for me and I am eager to help the team move forward and get some results through the remainder of the season."
Team boss Bobby Rahal said that he felt the change was necessary as the team try to rediscover the sort of form that won them the Indianapolis 500 with Buddy Rice in 2004.
"We have made important strides in our IndyCar program this year and we feel like this is something that we needed to do to maintain that pattern of improvement and growth," he said.
Simmons had raced for the team since Paul Dana's fatal crash at Homestead in 2006, and his best result so far this season was a sixth place at Texas.
Rahal added: "Jeff stepped into a difficult situation last season and did a lot for our team and our sponsors, but we feel at this time that this is the right move for the long-term future of Rahal Letterman Racing."