Thanks for the info, noseall. I was afraid that was the answer I'd get -- "retro-fitting throws up all kinds of problems"...
One issue I have is that I can't even find any good resources that show me how one would install a newel of this size even in new construction. I bought a few books...
I have an 1890 Victorian with mostly original woodwork. But a previous owner removed the original (presumably) stately bottom newel post and replaced it with a skinny, boring, chain store special.
I'm getting a custom 6" (150mm) turned post made based on the top newel, but I can't figure out...