Confused? I'd be surprised if you weren't.
In the original Back to the Future, which was released in 1985, the role of George McFly was done by actor Crispin Glover. However, in the two sequels, Back to the Future Parts II and III, the role was played by Jeffrey Weissman.
There are many people today that don't even know the role of George was recast. Why? The role of George in Back to the Future Parts II and III were made so that people wouldn't notice the difference.
When we first see George in Back to the Future Part II, it's just recycled from the original movie, when George and Lorraine see Marty with Jennifer and walk away, and this is right before Doc pulls up in Marty's driveway.
The second time we see George is in 2015, when old Lorraine and old George visit Marty and his kids. However, apparently, a flying car had fallen from the sky and hurt George's back and a machine was now used to hang George upside down.
This whole "upside-down" bit is actually used to hide the fact that that George is a different George, because it's not exactly easy to tell someone's features if they're upside down. Believe me, if George was right-side up, you'd immediately be able to tell the difference.
After that, George doesn't really have a big role. After Biff steals the almanac and gives it to his past self without Doc and Marty knowing, Doc and Marty go back to 1985. However, it's a different 1985, a 1985A, where Biff is rich, Lorraine is married to Biff, and George is dead. Pretty convenient how the person you can't get back for the sequel's character is dead in part of it.
Marty and Doc go back to 1955 and there, we see mostly scenes of Biff and Lorraine. At the Enchantment Under the Sea Dance, Marty sneaks in and looks through his binoculars and sees George dancing. This is one of the only times in the movie we get a clear shot of George's face, and it seems completely unaltered. Why? That was a clip from the original Back to the Future. Glover wasn't too happy about that, but I'll tell you more about that later.
The next time we see George is in the famous scene from the original movie where George knocks out Biff with one punch.
The video you see above is from the first movie. However, in the second movie, we see only the back of George. Now, you may think this isn't even that big of a deal, because we don't get a good look at Biff and Lorraine's faces, either. but after that, when Biff falls down, George helps Lorraine to her feet and they walk away. When he lifts up Lorraine, that's a clip from the first movie, and when they walk away, again, we only see George's back.
Later, Marty runs back into the dance and once again, only George's back is seen as he waves at Marty for helping him. After the Johnny B. Goode sequence is over and the other Marty goes to the back, we don't get a very good look at George's face. However, right before Biff goes, "You chicken?!" we get a VERY brief glimpse of George's face, through the window, which is where the picture is from (the pic you saw when you clicked on this). You'll see it's different.
Then, we only see George once in Back to the Future Part III, and when we do, he's in the very back in the shade, where we can barely see him clearly. He only has one line: "Lorraine, have you seen my glasses, where are my glasses?"
And here's what you actually came here for: why did they replace him in the first place?
If you haven't gone on Google and just looked it up by now, here's why: they didn't pay him enough money.
Here's a quote from Glover:
They offered me $150,000 to be in—it was a long screenplay. Like, a 200-something-page screenplay. I could tell they would split it into two movies. But Lea Thompson was making something like $650,000, and Tom Wilson was making something like $325,000 or $350,000, so it was less than half of what my fellow actors were making, coming back for similar-sized roles. And my agents knew it wasn’t fair. It wasn’t like I was saying I needed to make more money. I just basically, at that point in the negotiation, I just wanted to be fairly compensated.
Glover also said that writer Bob Gale had claimed Glover had asked for the same amount of money as Michael J. Fox, which wasn't true.
But there's more.
At the end of Back to the Future, George and Lorraine become very rich, which makes them happier. Glover didn't really like that message. He felt that the movie's moral shouldn't be money = happiness. It should be that love should be the reward.
Director Robert Zemeckis wasn't too happy about that.
To make it work without Glover, they cut down George's role and used footage from the first movie without paying him. They also used the molds of Glover's face from the first movie and put it on Weissman.
Now, Glover wasn't happy.
He filed a lawsuit against Zemeckis and won, which is kind of ironic because George isn't good at confrontations. New regulations were created that said they weren't allowed to use these kinds of methods (use another actor's clips without permission, use the face molds of another actor, etc.)
I guess the role of George just wasn't Crispin Glover's density.