The film credits about cast & crew on IMDb pages no longer show Nolan listed as executive producer. Has Nolan bowed out of the project? Or is there a role change?
This is correct.
Jonathan Nolan, Christopher Nolan's brother and The Dark Knight Rises screenwriter, gave an interview to Coming Soon where he said:
I love those guys and my brother was involved in ‘Man of Steel’ with Zack [Snyder] and David [Goyer]. I kind of feel like that chapter for us is closed. To be continued somewhere farther down the line? Maybe.
Whilst neither brother has given an interview explicitly stating why they've chosen to leave the projects, it doesn't appear like they were forced out - rather, it seems they left to pursue other projects.
In Batman Vs Superman Christopher Nolan and Emma Thomas will be executive producers, along with Benjamin Melniker and Michael E. Uslan.
Charles Roven and Deborah Snyder will be producers.
Batman v Superman: Dawn of Justice no longer lists Christopher Nolan as Executive Producer in multiple press releases since the initial announcement of the movie. Instead, the title of executive producer is being given to Benjamin Melniker, Michael E. Uslan, Wesley Coller, David S. Goyer and Geoff Johns. As stated by this press release by Warner Brothers featured on Businesswire