If Simon were to take the place of Piggy, would the whole story be different?
If I understand your question correctly, you are asking how the story would be different if Piggy didn't exist. If Simon, were Piggy, then Piggy would not be a part of the story.
I don't think the overall story arch would be that much different. Jack is still going to be a power hungry psycho backed up by lunatics like Roger. Ralph is still going to try and do what civilized people would do.
If Piggy were absent from the story as a character, the changes would occur early. It's Piggy who understands the basic application of science. For example, it's Piggy who figures out how to use the tools at hand (his glasses) to start a fire. Maybe the boys would never have gotten fire started without Piggy. I doubt that would be the case, but it would have taken them a lot longer. Piggy is also the character that teaches Ralph how to use the conch shell.
“How did your friend blow the conch?”
“He kind of spat, ”said Piggy. “My auntie wouldn’t let me blow on account of my asthma. He said you blew from down here.” Piggy laid a hand on his jutting abdomen. “You try, Ralph. You’ll call the others.”
If Piggy wasn't in the story, and Simon were to take Piggy's place instead, the conch sequence wouldn't have happened. Simon would have said something cryptic and mystical about the shell's beauty being both helpful and dangerous at the same time. Simon would have done the same thing with the fire situation. He likely would have wandered in from who knows where and said something like "if the island wants us to have fire, the island will provide."