I'm intending to buy my first miter saw for replacing hardwood flooring (termites), can envision adding crown molding down the road, and am leaning toward a 12 inch Bosch 5412L. The tool comments on Amazon and elsewhere seem to favor 10 inch saws as being more accurate (and cheaper!). So, my noob question is why can't a 10 inch blade be put on a 12 inch saw to combat deflection in specific circumstances?

I doubt that the downward motion of a 12" miter saw can be adjusted enough for a 10" blade to to cut completely through the wood. I also doubt that you will have enough deflection in a good 12" blade to notice.

I should go look at the saw in a store then. Do you consider the stock blade a good blade, or are you referring to replacing it with a Freud, Forrest, or other?

The stock blade, the one that comes with the saw, is pretty cheap, good for cutting 2x4s. I use the Diablo made by Freud for crown and trim. Good blade. I have a couple Forrest but they are for the table saw, great blades but expensive.