Windows 8 to Windows 8 Pro anytime upgrade is an excellent bit of kit. However, if you had ipv6 working before, and you added Pro to get the nice extras like Hyper-V (as I did), as soon as you install the Hyper-V role, all your lovely handcrafted ipv6 settings will be lost, and it seems to me that it also “lost” the jumbo frame support I’d enabled. This latter was because – it seems – that after installing Hyper-V, you’re running of a bridged network adapter. Now your original adapter which you’d set up jumbo frames on still (probably, mine did) have jumbo frames still enabled, the bridge adapter didn’t propagate the settings. Likewise, although it may have propagated the ipv6 address you set on your adapter, it’s probably forgotten to propagate them (especially, in my case the ipv6 nameserver and default gateway) across to the bridge.
So there you have it. Check ipv6 settings carefully after installing the Hyper-V role. This may be true of a native Windows 8 Pro installation; getting ipv6 working ahead of a Hyper-V role install may trigger the same problem, I really doubt that it was the Home->Pro update which caused the problem.