How is bicycle fit affected by switching from a riser bar to a flat bar?
Looks to me like the difference is as much in the stem as in the bar.  (It may be that for the Dr. Dew the bar has simply been rotated to a flatter position.)  And both bikes appear to use a threadless headset, so the stem can be replaced without touching the headset proper.  In some cases the stem is designed to allow it to be removed from the bar without having to "strip" the bar, in other cases not.  Can't tell from the pictures.

Minor differences in bar position -- height and "reach" -- can have a major effect on riding comfort.  Lowering the bar and/or moving it forward will significantly increase the pressure on the hands and add strain to the back.  It does improve aerodynamics significantly, but, unlike with a "drop" bar, you tend to be stuck with that lower position, even when you'd like to relax a bit.  (But there are various add-ons for straight bars that allow one to switch back and forth between a relatively upright position and a more aero one.)

(In fact, it's a pet peeve of mine that many bikes are designed to look "mean" on the showroom floor, with bar positioned low, but the bars are positioned too low for the comfort of most riders, and the threadless scheme makes it harder to raise the bar very much.  As a result, many bikes are purchased, ridden for a few weeks, found to be uncomfortable, and then forgotten.)