When and why was the ending changed?

How could the original ending have been changed, decades later? The only way that would have been possible is if they had filmed two endings in 1956 and later cut out the "original" one for the "substitute", and there is no proof this ever happened. I have never seen an ending to this film other than the one shown. It may be different from what's in the novel but the movie is the same.

The ending seen on TCM is the same one that I've seen since I first saw the film in 1967 on TV. Even the book "The Films of Humphrey Bogart" which was published in 1965 reports the ending as shown.

It ain't easy being green, or anything else, other than to be me

I think the other responders misunderstood the wording of your question.

Yes, of course, The Harder They Fall has had the same ending since its release in 1956.

But according to United Press, April, 1956, the Bogart character's article originally began "Boxing should be outlawed in the United States if it takes an act of Congress to do it."

That was reshot at Schulberg's request to read "Racketeers in boxing should be outlawed if it takes an act of Congress to do it." And, yes, it was because he was calling for the reform of boxing and the banishment of its ruling criminal element (e.g., Frankie Carbo, Blinky Palermo, Jim Norris), not its outright abolition.

That view was in stark contrast to the film's director, Mark Robson (who also directed Kirk Douglas's starmaking boxing classic "Champion"), who made no attempt to hide his disdain for boxing, in a number of interviews.