Windows 8.1 was released to the Windows Store last week and aside from having to download the same 3.9 GB update on each machine I've installed it on it's been a relatively easy and smooth update. I did, however, run into one small problem related to Oracle VirtualBox. When you install VirtualBox a "Host-Only" network adapter is installed in Windows to allow guest VMs to communicate with the Windows host, as the screenshot below shows:
Unfortunately the Windows 8.1 upgrade process deletes the network adapter. After the upgrade you'll be faced with a warning message in the VM properties window and your VM won't start.
The fix is easy - just reinstall VirtualBox. It's not a bad idea, actually, as at the same time as Windows 8.1 went live so did VirtualBox Version 4.3. The latest version has a few nice improvements that make it worth the upgrade, including: Multi-touch support, virtual USB webcam, a new NAT adapter that enables hosts and guests to communicate without requiring a host-only adapter, and the ability to record the entire VM session to video.
As always, if you upgrade VirtualBox it's a good idea to upgrade the guest additions as well.