Windows 10 license is now tied to your digital account (i.e. MSN) and there's a good chance now that simply moving your Windows drive to the new motherboard will work perfectly.
Not trying to push AMD but that does seem like a better option for the price. Sure performance in some games will be better on the i7-4790K but you get 50% more cores/threads with the R5-2600 which can help game smoothness and FPS in other games (especially as games need more CPU in the future) plus AM4 isn't dead yet and the Zen2/3000 series should be compatible.
*In fact, I'd consider doing what I said above but get an R5-3600 CPU instead later this year (which may be roughly 20% faster than the R5-2600).