Judges, the people who arbitrate copyright disputes in the end, are humans, not computer programs. I don't think that if you tried to rip off Yoda, but made slight alterations to his face wrinkles, that you're going to convince a judge that it's something other than Yoda. A judge is not going to see Yoda as anything other than the same character through all six movies, no matter how many differences between the 3D model in the Phantom Menace and the 3D model in Attack of the Clones a "diff" command might find.
A direct copy, would be the _exact_same_ characters as depicted in ep VI, but alas they are not, they are however similar to, which makes it a derivative of.