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Interestingly, in John Ford's 1940 movie version, the only significant change was the ending. The studio would not let the movie end with some old dude drinking breast milk from Rosasharn after the miscarriage. Instead they have Ma Joad give a rousing speech about family and never giving up. So if you want a happier ending, you could consider that.
The ending is pretty depressing but then it is Steinbeck after all so I would not change it.