UPDATE 1 while I was writing this post, I started to enable Media Center with my previous key which has been validated before being able to continue. The wizard failed at the end with an unknown error and when I retried with the key the validation failed. WTF!!!!!
UPDATE 2 after some reboots, Media Center is now installed. The process is clearly unstable as the wizard to enable this feature failed.
Following the availability of Windows 8.1/Windows 2012 R2 for MSDN/TechNet subscribers, I started to upgrade my environment by upgrading my laptop running on Windows 8 Pro with Media Center.
Even if the upgrade went smoothly – no issue at this time, I discovered that Media Center has disappear while I asked to keep all files, applications and settings during the upgrade 
Fortunately, it seems that the key for Windows 8 Media Center still works:
- To re activate Media Center, just go to the System Settings
- Click on Get more features with a new edition of Windows
- Then just select I already have a product key
- And enter the product key you get to enable Media Center on Windows 8 Pro
This is really annoying to do an upgrade and being obliged to re activate an existing and installed feature of Windows 8