The first signs of trouble between Bochco and ABC surfaced last week in Broadcasting & Cable, which reported that Bocho was unhappy with network suits giving him too many notes on the show.
A report on Entertainment Weekly's Web site followed, saying that Johnson, who's been with the show since its premiere, will replace Bochco as showrunner.
Bochco replaced original showrunner Rod Lurie last fall.
Davis stars in "Commander" as the country's first woman president.
The show got off to a strong start last fall, averaging 16 million viewers, but has lost steam and was yanked from ABC's schedule last month. It's scheduled to return April 18.