I have a surface Pro with Windows 8, and also a virtual copy of Windows 8 in virtual box.
Thursday afternoon I started the upgrade to 8.1 in virtualbox and ran into a little problem the windows store said my CPU was not compatible. I then began to search the problem upgrading Windows 8.1 and virtualbox and found out I just had to change the operating system version in virtualbox setting to 8.1, went back to the window store and I was able to upgrade without any problems after that.
Friday morning I started on the surface Pro 8.1 upgrade, went to the window store and started to download, a short time later 8.1 was installed without any problems.
There was only two programs that I had a reinstall was avast free and sandboxie, and just minor Metro tile fixing I had to do.
But overall the upgrade was a success for me.