I'm not sure if you've seen it so I'll give you the basics assuming that you have not seen it. Wayne's character was an A**hole! I thought it was the best performance he ever gave honestly. But the character and his son (Montgomery Clift) were at each others throat the entire movie. At the end of the movie they got into a fight and they basically had a smile and hug feel good moment at the end. My understanding is that Clift's character was suppose to kill Wayne's character. The studio didn't want Wayne dying and they sure didn't want movie goers seeing him as a villain so they had this radical personality change at the end. I like to say that the movie suddenly turned into McLintock! at the end.
Wasn’t the studio that changed the ending, it was Howard Hawks. He never liked the idea of his characters getting killed at the end. As his career went on, he got more into the characters and less what the plot “should” have determined.
I like the ending of Red River because it defuses all the phoney hate between the two. It’s actually a very advanced narrative idea. By the time Hawks got to Rio Lobo, though, he’d completely lost interest in coherent storytelling. But as late as El Dorado, he was making terrific movies.