It's just that, while learning how to do these home repairs, some mistakes can turn out pretty costly (if they can't be remedied!)
Well, the 2 1/8" hole is drilled (fairly straight...) and the mechanism fits just fine. Unfortunately, I am unable to get the hole saw off the drill bit to change it! The mandrel and adaptor are stuck tight to the 2 1/8" hole saw. So I cleaned everything up and will get back at it tomorrow.
Thanks so much for your help and suggestions. It would be easier to learn with a "teacher" right here with me, but the Internet (and BV bulletin board!) are the next best thing!