The phrase "fell up on" does not have any established antonyms. This is because the word "fell" indicates a sudden drop, stumble, or collapse. "Up" conveys the idea of advancement or progression in a vertical direction. These two words create a contradictory image, making it hard to find antonyms that will provide the same meaning. However, there are several words that could give the opposite meaning of either "fell" or "up," such as rise, ascend, climb, ascent or topple, descend, collapse. These words capture the sense of motion or movement in a different way and provide an alternative contrast.