I would keep what you have. The wood color goes better with your flooring, and fits your house better.

We're going to be redoing all the flooring in the house, I'm just not quite sure what extra costs doing the stairs entails.

Impossible for anyone to tell. Too location dependent but it will not be inexpensive. Would not surprise me if it were more than having the floors refinished.

Perhaps I'm missing something but it looks like the only differences are the stair, newels and handrail color and paint on the risers and skirt board. Pricing would depend on where you are but I'd guess you're looking at a minimum of $2500.00 and maybe a maximum of $4500.00 just to do these items. A floor refinisher would do the treads and floor, a painter could do the newels, handrails and the painting.

Would not surprise me if it were more than having the floors refinished.

I agree. I'm trying to figure out how it is possible to strip, sand, stain & finish that center post in the newel assembly. Plus there is the railing visible going to the basement and up top on the second floor....

It is not necessary to strip, sand, stain and finish the newels or handrails when going darker. It could be done by a painter using a brushing stain rather than with a penetrating stain used on flooring. That said, I don't envy the person doing the newels at the bottom, which would be a lot easier with the lower balusters removed.