How can I replace the star-shaped base of an office chair?
Ok, I did it.

Just dumped some WD-40 and let it soak for 1 minute, then I grabbed my good ol' hammer (I don't have a rubber mallet) and started gently but commitedly striking the edge of the post, with the chair turned upside down, while I grabbed any of the star's legs. It slipped off and then I just hammered its way in into the new post, good as new.