For the most part, replacing the motherboard requires a reinstall. Certain chipset drivers are installed that are based on which motherboard is installed, and are hard to replace without a reinstall.
About the only time you can get away with not reinstalling Windows is if the motherboard is replaced with an identical model, which will use the same chipset drivers.
How about saving my important programs and data like Photoshop,Steam and games,and pictures? I have an external usb hard drive. Should I copy that to it? Or can I somehow make a copy to my existing HDD before I change everything out? What would be the best procedure? I will be using a new case too on this build. I guess I can put in a fresh install of Windows 7 on new HDD and then plug in old hard drive and transfer files I need over to new HDD? Or will that confuse everything since my old HDD will still have Windows 7 on it also? I also am using a very good registry edit/repair/clean program. Will using that program help me avoid any conflicts?