After Finny's leg has recovered enough for him to return to Devon School, he appears to be different in several ways. First, he seems to be solidly against the war. Before his injury, he constantly wanted to compete with the seniors who were conditioning for the war and even discussed enlisting early. When he returns and finds out that Brinker and Gene have talked about enlisting, he is flabbergasted and completely against the idea. He makes comments about the futility of the war and how he hates the thought of it.
Secondly, in the book's beginning chapters, Finny cares nothing for records or winning. He beats the school's swimming record but doesn't want it to be recorded. In contrast, when he returns to Devon, he begins diligently training Gene for the Olympics because he cannot go himself.
Gene thinks that this is a different Finny, but as Finny lies in the hospital with his second broken leg (near the book's end), Gene finds all of the letters Finny had been sending to the world's armies to see if he could join them since he knew the US would not take him. With his discovery, Gene realizes that all of the seeming contradictions of the "new" Finny are because Finny still wanted so badly to be part of the war effort. He didn't want Gene and Brinker to enlist because he couldn't. He critiques the war because it hurts too much to admit that he cannot be where the action is. So in essence, Finny does not have a new lifestyle--he is hiding his old desires.