I would have to say that WMC isn't exactly the conventional definition of an upgrade. Windows seems to separate that into two categories.
A generic upgrade normally refers to installing a more recent version - Windows 96, XP Vista Win 7 - upgrade to Win 8.
Windows 8 regular is shipped pre-installed on computers. There is a stripped down version of Win 8 for notebooks and tables, smaller devices, touch screen.
Microsoft is offering Windows 8 as a download on line with no disc for $39.95. Promo ending soon when the price will jump to $199. I have read about many problems with it. I suspect that it may be a "stripper" version, not full.
Windows 8 Pro can be ordered on line from many vendors who mail, or send, the box of two discs 64 bit and 32 bit. Most stores charge $69.95. That Promo will end soon also.
For anyone who bought a computer with Windows 8 factory installed, that can be upgraded from Win 8 regular to Win 8 Pro. The Pro version is more for small businesses, government, hospitals and such. It has enhanced encryption and a couple of other somewhat obscure features that the average home owner, private user, or casual user, won't need or want - probably. That will cost $69.95.
For anyone who has a new computer with Windows 8 pre - installed, if they want WMC Windows Media Center, they will be required to upgrade to Windows 8 Pro at a cost of $69.95
For anyone who has upgraded from Windows 7, Vista, XP to Windows 8 Pro, and already paid the $69.95, the WMC addition may be free. There was a rumble about charging $9.95 which may not actually be in effect.
So it seems that the WMC offer is something or a hybrid betwixt and between an upgrade and an update. Conventionally I would call it an optional update. An upgrade is paid. An update is free.