Basically the animators had access to the original drawings and traced the new characters on top frame-by-frame using a light table.
It’s a variation on the use of “rotoscoping.” Still a lot of work, but less work than trying to parse out the nuances of character motion from scratch.
“They all said it wasn’t possible to make an entire episode of an animated TV series with a budget of just five drawings, but we sure showed them!” —Cambria Productions
Pixar had some fun with that style of animation with The Adventures of Mr. Incredible & Pals, included as a bonus feature with the DVD of the movie.
Apart from the repetitiveness of the topic I think that this discussion is an interesting one to have.
My take would be that in both CGI and hand cell animation the ethos has always been to reference ‘character’ motion from real life or other animated source material… watch any PIXAR ‘making of’ and you’ll see an example of this. I’d imagine in a time consuming process in a studio production environment (economically driven) that they would find it more expedient and money saving to replicate motion of a scene if it has been done before… Why reinvent the wheel?
I’m thinking if this is the case then there’s a whole new can of worms to be opened re intellectual property rights!!