I currently dual-boot Windows 8.1 ProWMC and Server 2012R2 - however, I am soon going to upgrade both motherboard and CPU - is a reinstall of both OSes a must, or can I reinstall just one (from the other)? Current motherboard: ASUS P5G41M-LX2/GB Current CPU: Intel Core2Quad Q6600 (Kentsfield) Target Motherboard: MSI Z87-G41 PC Mate Target CPU: Core i5-4670K (Haswell) While a complete reinstall would be a pain, if needed, it's doable. (I followed the rules of what NOT to back up, and no irreplaceable data will be lost.) What NOT to back up (to the cloud or any other sort of way): 1. Motherboard or other drivers not needed for OS installation. 2. Non-basic drivers for any hardware. 3. Easily-replaced software. 4. OEM software - nine hundred and ninety-nine cases out of one thousand, such software falls under #3. 5. Subscription software - especially MSDN/TechNet. (MSDN/TechNet software available as part of a subscription now falls under #3.)
You would have to re-activate, but as far as booting... Well, I'm not entirely sure. You might have to load up a bootable disc so you can run bcdboot c:\windows and then modify the existing boot entries... or not... I don't really know. Most people re-install the OS when replacing most of the hardware.
To my experience, in opposite to Win 7, Win 8.x is very picky with that. You can try, but be prepared it might behave unpredictable. When starting the old system, in case it doesn't die with BSOD, give it time to check and install default drivers first (this might need a while), then go ahead .