Here are where the answers generally came from: http://www.microsoft.com/OEM/en/licensing/sblicensing/Pages/licensing_faq.aspx#fbid=Ea6S5dihcdH
1: For the first time activation it will work the same way as any first time activation. Online activation should work but if not the one time key will be displayed to you to provide to the telephone automated system or operator. If activated more than once though more than likely you will need to use the telephone system.
Q. Will the downgraded software require product activation? If so, what product key should be used to activate the software?
A. Once the downgraded software is installed, the PC will prompt for a
product key in order to activate the software. The product key
associated with the original Windows software should be used for
activation. If the product key has been previously activated, which is
likely if the media came from a prior legally licensed version that
has been activated in the past, the software may be unable to activate
over the Internet, due to the hardware configuration change when
installing this media onto the Windows 8 or Windows 7 system. When
this happens, the appropriate local Activation Support phone number
will be displayed, and the person performing the downgrade will need
to call the Activation Support Line and explain the circumstances to a
customer service representative.
2: Yes. It can be done with any legal media except OEM. OEM media is tied to the specific workstation since the license is "tied" to the motherboard of the workstation
Q. Can a PC with an OEM Windows operating system have its motherboard upgraded and keep >the same license? What if it was replaced because it was defective?**
A. 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
those terms. The End User Software License Terms are a set of usage
rights granted to the end user by the PC manufacturer, and relate 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.
3: No. MS will not issue a new key because there is no need. The activation key may be different but the actual product key is what is used for the downgrade.