If you've seen Rogue One, you probably noticed that much of what's in the trailers is not in the movie. That's because the movie took a lot of time reshooting and reworking its third act. However, none of that comes close to how different the ending of the original script was. An ending that was not even shot.
In a post-release interview with Empire, director Gareth Edwards explained how the first version of the script was completely different from what we now see in theatres. Obviously, big spoilers follow:
Speaking with Empire, Edwards was asked if the end of the film -- where all of the main characters die -- was always the end of the film. It wasn't. Here's his response:
The very first version, they didn't. In the screenplay. And it was just assumed by us that we couldn't do that. 'They're not going to let us do that.' So I was trying to figure out how this ends where that doesn't happen. And then everyone read that and there was this feeling of like, 'They have got to die, right?' And everyone was like, 'Yeah, can we?'
We thought we weren't going to be allowed to but Kathy [Kennedy, President of Lucasfilm] and everyone at Disney were like 'Yeah it makes sense/ I guess they have to because they're not in A New Hope.' And so from that point on we had the licence.
I kept waiting for someone to go, 'You know what? Could we just film an extra scene where we see Jyn and Cassian, they're ok and they're on another planet?' And it never came. No one ever gave us that note, so we got to do it.
I think I speak for most people when I say, I'm glad they made this decision. The film's ending as it stands is much more powerful and unexpected.
Edwards talks about those extra shots, as well as putting Tarkin in the film, in the Empire interview, which you can listen to in the first 20 minutes of the below link.