Broadwell should work in current 9-series motherboards while Skylake will likely require a new motherboard.
The mobile variant of Skylake will reportedly debut just before the desktop chip. 15W and 28W flavors of Skylake-U are on tap, both with BGA packages. There's no mention of a lower-power replacement for the Core M, though. The lowest-wattage chip on the roadmap is the Braswell replacement for Bay Trail-D, which is supposed to arrive early in Q2. That SoC is listed with a 10W thermal envelope, and it seems to be targeted at low-power desktops. Intel is already shipping Cherry Trail, its Bay Trail successor for tablets.