Tom's philosophy changes in recognizing that mere survival does not represent what he wants out of life. This change results from his work with Casy and internalizing Casy's teachings into his own personal ideology.
When the novel opens, Tom is a bit distant from the rest of the world. Part of this might be due to the four years he has spent in prison. Another aspect might be in his individualistic frame of reference. Survival and endurance are the elements that defines Tom's existence. While Tom assumes leadership of his family, helping them as best as he can, he remains a bit apart from the world around him.
I think that Tom changes from his time with Jim Casy. He sees his inherent goodness. Tom recognizes Casy didn't have "an angle," possess only a truthful belief about the universality of human beings. Tom embraces this philosophy of life. Part of the reason he does is because he witnesses how Casy sacrifices for his values in the cause of helping others. As Tom bears witness to this, it leaves an impact on him. Tom says as much to his mother:
Says one time he went out in the wilderness to find his own soul, an’ he foun’ he didn’t have no soul that was his’n. Says he foun’ he jus’ got a little piece of a great big soul. Says a wilderness ain’t no good, ’cause his little piece of a soul wasn’t no good ’less it was with the rest, an’ was whole.
Casy's philosophy has embedded itself on Tom. He understands that survival is not as important as embracing something larger than oneself, to be a part "of a great big soul." His own value system begins to grow as a result.
With a new set of values, Tom is able to see himself more as like Casy. He commits himself to living his life as one who is more concerned about others, even more than himself:
Wherever they’s a fight so hungry people can eat, I’ll be there. Wherever they’s a cop beatin’ up a guy, I’ll be there. If Casy knowed, why, I’ll be in the way guys yell when they’re mad an’—I’ll be in the way kids laugh when they’re hungry n’ they know supper’s ready. An’ when our folks eat the stuff they raise an’ live in the houses they build—why, I’ll be there.
In many ways, Tom sees himself as Casy's "spiritual heir." He wants to take the work Casy was doing and expand upon it. Tom admits that his wordss sound like he is " talkin’ like Casy." He has inherited Casy's ideology into his own. He moves from an individualist to a more collective notion of the good. Tom notes this change takes place because he is "thinkin’ about him [Casy] so much."