The siding should easily be able to be cut back to have the flat casing installed. It appears for what ever reason that the double hung windows have the flat casing and the casements/fixed transoms do not, which I feel is typical. That being said on the same house I would probably want the windows all trimmed the same.

Thanks - any idea of ways to "easily cut back" the siding? My husband is pretty handy and I know this is something he'd be willing to tackle himself having already replaced a few clapboards on other sides of the house. But, how does one saw out the clapboards without damaging the sheathing? Is there a fun blade attachment I can buy him?

Just be sure your husband protects his fingers and other important body parts.

when I do something like this, I add a "scab" board to act as a straight edge for the saw. Something like a piece of your trim, with a single nail at each end tacked to the siding. That way I can concentrate on depth and make multiple passes, getting progressively lower each pass never cutting all the way though the siding where it meets the sheathing on the house. Then snap away the cut piece and trim the small remaining piece of siding with a razor knife.

Helpful responses here.

OP, I just wanted to tell you that DH and I built our house, after it was framed, mostly on weekends, and DH installed all the board and batten siding on top of a laborious Fine Homebuilding suggested rain screen. I started out wanting super-contemporary minimal trim, then quit paying attention as I began painting. As a result, our windows have a similar eclectic trim profile which drives me crazy if I pay attention. So I don't.

But I feel your pain. Good luck with your very cute house!

Thank you all!