If the Windows 7 is an OEM copy, then the general opinion is that that license MUST stay with the old machine - Windows 8 Upgrades can be moved but must be uninstalled from the old machine and then used to upgrade a different license on the 'new' machine.
Full Windows 8 licenses can be moved from machine A to machine B - you must still uninstall the license from machine A, but can clean install on machine B

My understanding is that the Media Center pack is not movable between machines - but since I hav no use for Media Center currrently, I haven't gone into any detail on it.
Don't forget that Win8 Pro comes with Media Player - just not the fancy interface of Center.