In majority of the cases, immaterial of animated or live action movies, the conversion from non-stereo to stereo adopted is the same. i.e post conversion. But yeah, Pixar's Finding Nemo and Monster's Inc. were an exception. They choose to entirely recreate their movie in true stereo, and that comes at a huge cost. In the span of 10yrs, from the original release of Nemo in 2003 to the time when they decided to recreate the entire movie for stereo, Pixar's softwares to create movies had very sophisticated update. So to make things work with the current generation softwares, Pixar took 9 months. And another 9 months after that to re-create the stereo version. So they did it in 18 months, where as a post stereo conversion takes only a couple of months and in a fraction of the cost. Even we at DreamWorks Animation Studios wanted to do the Shrek(1/2/3) series as a true stereo, but the budget was way too high, even though our software updates were not as sophisticated as Pixar's. So yeah, I guess no other studio would dare to invest in an actual re-creation of a non-stereo movie to true stereo like Pixar did.
Cheers.