Good: Martin Lawrence, Bad Boys II
Let's get the sequel conundrum out of the way first. Back in 1995, Will Smith was not yet a proven commodity, so he was second-billed to Martin Lawrence, and the plot revolved around Lawrence's character; Smith was clearly the sidekick. Eight years later, Smith was a worldwide box office star, and Lawrence was… not. Smith took on the lead role, with the plot revolving around him, and Lawrence was pushed back to secondary, sidekick position. It's a significant change that's pulled off with subtlety -- the only subtle thing in the movie!