According to this article, the 4K release of Toy Story has been upscaled from the original resolution it has been rendered at. They also cropped the video instead of letter-boxing it.
If it were shot on video (not film which can be re-scanned) I could understand why Disney would choose to upscale it, however this is a CGI movie and they still have the original files. In fact, according to this Quora Q&A, they have already re-rendered it at some point in time according to a Pixar employee. In addition, the Wikipedia article summarises how they created a second camera for each shot using the original files. Because of this, I don't understand the reason they upscaled the movie instead of re-rendering it.
Edit: I received a comment asked if they had the original files. I've already mentioned this but you can read here in this New York Times article.
The process of taking the original files from the first two movies and getting them to a place where they could be enhanced was one that Mr. Lasseter called “digital archaeology.” “We had to have some very, very smart people at Pixar go back in and write some software and figure out a way to make it so that those files would render on our current computers,” he said.
Emphasis added. This was done for the 3D re-release so they could have built off of what they did for that to make a 4K re-release. They've already put the work into it. That's why this baffles me so much.