It's worth noting that you can only replace the MB with the make/model or equivalent (I.E. Not an upgrade). Upgrading the MB makes the system a new computer in the terms of Microsoft licensing. This only applies to OEM licensing.
From the Microsoft Licensing FAQ
http://www.microsoft.com/oem/en/licensing/sblicensing/pages/licensing_faq.aspx#faq3
Generally, an end user can upgrade or replace all of the hardware components on a computer—except the motherboard—and still retain the license for the original Microsoft OEM operating system software. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created. Microsoft OEM operating system software cannot be transferred to the new computer, and the license of new operating system software is required. If the motherboard is replaced because it is defective, you do not need to acquire a new operating system license for the PC as long as the replacement motherboard is the same make/model or the same manufacturer's replacement/equivalent, as defined by the manufacturer's warranty.
The reason for this licensing rule primarily relates to the End User Software License Terms and the support of the software covered by that End User Software License Terms. The End User Software License Terms is a set of usage rights granted to the end user by the PC manufacturer and relates only to rights for that software as installed on that particular PC. The system builder is required to support the software on the original PC. Understanding that end users, over time, upgrade their PCs with different components, Microsoft needed to have one base component "left standing" that would still define the original PC. Since the motherboard contains the CPU and is the "heart and soul" of the PC, when the motherboard is replaced (for reasons other than defect) a new PC is essentially created. The original system builder did not manufacture this new PC, and therefore cannot be expected to support it.
