Last-minute genre shifts can be awesome, indeed. (Or not. Having linked to one Family Guy clip in my other post, let me link more positively to this one as an example...) I suppose it depends how firmly the tail is attached. You need a strong tail to wag a whole dog, after all.

rachelmanija's distinction between genre shifts and tonal shifts is an interesting one, and might apply to your Henry James examples. But is it a difference of degree or kind? That I'm not sure of...