You could use get stain, since you're going darker. But you need to make sure you clean the old finish thoroughly, since waxes, oils, etc., will cause problems. And you'll have to apply in very carefully to get an even coat (or two), which might be tricky with all the turnings on your chairs. You might also need a top coat of polyurethane or something similar, to protect the finish from scratches.

Thanks so much for your info. I'll look into the gel stains, that would something new to me. And I do understand what you mean about the thorough cleaning before application. Fortunately, the two styles of chairs that I intend to treat do not have so many turnings. And I figured I would need a top coat for protection. I appreciate your time and interest.