Here is what the page “Why Conquest of the Planet of the Apes Changed Its Original, Much Darker Ending” contains. The 1972 sci-fi sequel Conquest of the Planet of the Apes originally ended with a far bleaker climax, but test audiences and studio pressure led to a last-minute rewrite. The original finale saw the ape leader Caesar brutally killing Governor Breck and declaring war on humanity, a conclusion directly inspired by the 1965 Watts riots and the 1968 assassinations of Martin Luther King Jr. Screenwriter Paul Dehn intended the grim ending as a cautionary allegory for racial and social unrest in America.
However, after a preview screening in Phoenix, Arizona, audiences reportedly cheered the violence, alarming executives at 20th Century Fox. Jacobs agreed to soften the ending, having Caesar spare Breck and deliver a more hopeful speech about coexistence. For the complete material, the listing points back to the source inside the network.