It wasn't a new product, they kept a lot of the assets (notably the characters and the universe) but rewrote the script extensively and had to scrap a lot of animation.

I don't think an analogy to software rewrite works very well, the domains are so different. For example Joel makes the point that code have a lot of embedded information which you have to rediscover if you rewrite from scratch. This introduces a huge risk and uncertainty.

If you rewrite a movie script halfway during production you have to scrap a lot of work but you don't really loose knowledge, so the risk is more manageable.