I've seen questions about Piggy changing answered in two ways.  First, I have seen it defended that Piggy doesn't change over the course of the novel.  Piggy begins the book as the voice of knowledge and reason, and he is still this character when he is killed by Ralph.  He's obsessed with rules and order, which can be seen in his repeated defense of the conch.  He's serious throughout the book, and he adamantly feels that the world should work on a fair justice system.  

“Well, that isn’t fair. Don’t you see? They ought to do two turns.”

Other characters in the book change, while Piggy maintains his evenness.  Ralph loses confidence and power throughout the story, and Jack becomes more powerful and sadistic as the novel progresses.  Piggy simply can't understand why the other boys can't act like adults.  He knows their actions are wrong, and he doesn't understand how their sense of "right" has been so quickly abandoned.  

“Which is better—to be a pack of painted Indians like you are, or to be sensible like Ralph is? . . . Which is better—to have rules and agree, or to hunt and kill?"

However, I do believe that chapter 8 does show some small changes in Piggy's character.  Before chapter 8, Piggy knew that the other boys were not likely to listen to him or his ideas.  He knows that he is smart, and he knows that his ideas are good; however, he knows that the other boys simply won't agree with him.  They only see him as the fat kid with asthma.  That is why Piggy quickly latches on to Ralph.  In a sense, Ralph can be Piggy's voice.  It's Piggy who explains to Ralph why the conch is useful and how to use it.  

“We can use this to call the others. Have a meeting. They’ll come when they hear us—”

He beamed at Ralph.

“That was what you meant, didn’t you? That’s why you got the conch out of the water?”

Before chapter 8, Piggy is more or less happy to be in the background.  His ideas and his voice are heard through Ralph; however, that changes in chapter 8.  Jack has finally had enough of Ralph’s inaction, and he leads a group of boys to another section of the island and declares himself chief.  Ralph is shocked into complete inaction.  He is stunned that everything has fallen apart so completely.  It’s Piggy that now speaks up and starts giving orders.  He even rebukes Ralph.  

Piggy was indignant. “I been talking, Ralph, and you just stood there like—”

Piggy then announces a solution to not being able to have a fire on the mountain.

Piggy was speaking now with more assurance and with what, if the circumstances had not been so serious, the others would have recognized as pleasure.

“I said we could all do without a certain person. Now I say we got to decide on what can be done. And I think I could tell you what Ralph’s going to say next. The most important thing on the island is the smoke and you can’t have no smoke without a fire.”

Then a few lines later, the text says the following about Piggy:

Piggy lifted the conch as though to add power to his next words.

“We got no fire on the mountain. But what’s wrong with a fire down here? A fire could be built on them rocks. On the sand, even. We’d make smoke just the same.”

Notice how Piggy is speaking with confidence.  He’s attempting to finally take some control of the island situation.  Also, notice the mention of the conch.  When he first came across the conch, Piggy handed it to Ralph because Ralph was more likely a better power symbol.  Now, Piggy is trying to put on that mantle.  

Some of the changes in chapter 8 are not Piggy changing, but the society on the island changing around him. However, Piggy himself does change some. He becomes crueler, speaking unkindly to Simon. In doing so he aligns himself more with the shift that's happening in the boys on the island, and less with the kind, civilized Piggy who existed before. Rather than just explaining what should be done, or sharing ideas, Piggy shows impatience by complaining that the other boys don't understand why the fire is importance. This shows he is stressed and tired.