Marvel has done an amazing job of keeping their actors. There haven't been any major recastings in years, but two heroes were replaced at one point that had fans scratching their heads. First, Edward Norton was replaced by Mark Ruffalo as the Hulk. Then, Terrance Howard was replaced by Don Cheadle as War Machine. 

Terrence Howard has opened up about being recast in the past, claiming that Marvel Studios offered to pay him a fraction of what they promised for the sequel to Iron-Man. Howard was one of the highest paid actors in the first Iron-Man film, but he claims that Marvel told him they knew the franchise would do well with or without him, so they rescinded on their original deal and offered him less money to return. Howard, obviously a man of principle, fought to get his promised money, but failed. The Empire star recently sat down on Watch What Happens Live with Andy Cohen, and answered yet another question about his return to the MCU. 

"You know what’s so funny?," Howard replied to a fan that called in and asked about the MCU "Even though I love Don Cheadle so much, and I love what he’s done, I still hear a lot of fans asking, 'Am I going to come back and be War Machine?' Yeah? Am I going to come back and be War Machine? I think they could have a huge franchise off of it, but fuck 'em."