If I changed from a Gigabyte B85-MD3H to a ASUS Z87-K. Do I need to reinstall windows?
Socket LGA1150
I would personally reinstall just to avoid any driver conflict with the motherboard but before any of that I would also suggest you check which version of windows you have because I have heard of some version like the OEM only being able to active on the same motherboard or hard coding it self to that motherboard.
I'd reinstall as well. And yes, an OEM version of windows will only work with the same type of motherboard as previous whereas with a retail you should be able to migrate it from board to board.
I didn't when I switched my mobo, no issues here. I just removed the old software and such, made sure chipsets appear correct in the device manager. Not sure if I'm taking a performance loss- I haven't noticed any issues.
Should reinstall with any major hardware change, hope you didn't buy an OEM windows key
A new motherboard, by Windows definition, is a new computer. At the very least you'll need to call MS support to have it reactivated if you avoid the route of reinstalling.