I don't believe that this was reusing old cells or anything like that. Many scenes in Disney animated films were first shot using live actors on a sound stage. The animators would then draw the characters going frame by frame looking at the actual stance of the live actors. So, this was more a case pulling out the old live action footage as a guide once again. The actual hand drawn animation for each film was totally new stuff.
http://studioservices.go.com/disneystudios/history.html scroll down to the 5th picture of the live action sequence for lady and the tramp.
RE: Sleeping Beauty
Live-action reference footage
Before animation production began, every shot in the film was done in a live-action reference version, with live actors in costume serving as models for the animators. The role of Prince Phillip was modeled by Ed Kemmer, who had played Commander Buzz Corry on television's Space Patrol five years before Sleeping Beauty was released. For the final battle sequence, Kemmer was photographed on a wooden buck. Among the actresses who performed in reference footage for this film were Spring Byington, Frances Bavier, and Helene Stanley.
Helene Stanley was the live action reference for Princess Aurora. The only known surviving footage of Stanley as Aurora's live-action reference is a clip from the television program Disneyland, which consists of the artists sketching her dancing with the woodland animals. It was not the first or last time Stanley worked for Disney; she also provided live-action references for Cinderella and Anita from One Hundred and One Dalmatians, and she also portrayed Polly Crockett for the TV series Davy Crockett, King of the Wild Frontier. An episode of The Mickey Mouse Club television series features Stanley re-enacting scenes from the Sleeping Beauty for the Mousketeers to watch (a clip from this episode is included as a special feature on the Cinderella Platinum Edition DVD).
All the live actors' performances were screened for the animators' reference as Walt Disney insisted that much of Sleeping Beauty's character animation be as close to live action as possible.