Did the update via Windows Store. Now:
(1) Comodo Firewall isn't loading, can't uninstall it, can't reinstall it ("This program has compatibility issues")
(2) Avast didn't survive the update, not even installed, can't install it ("This program has compatibility issues")
(3) Belkin WiFi USB adapter drivers won't load ("This device is not working properly because Windows cannot load the drivers required for this device.")
The third one is the biggie: No internet connection. The same Windows 7 drivers that worked with Windows 8.0 won't work with Windows 8.1
I suspect things might be smoother with a clean install, but as I understand it we common folk with no MSDN or Technet subscriptions don't have that option?
As a follow up: To get as close to a clean installation as I could, I deleted the Win8.1 partitions and reinstalled Win 8, with no apps installed, and minimal configuration (only Folder Options to show hidden files, all file extensions, and permit single-click.) I also installed (and was able to use) the Belkin USB WiFi adapter using the Win7 drivers.
Then I ran Windows Update twice to install >1GB updates and then upgraded to Win8.1 via Windows Store.
The Belkin WiFi adapter will install the Win7 drivers, and there is no problem indicated in Device Manager. However, Windows Networking apparently doesn't recognize it and won't create a WiFi connection, so basically the WiFi adapter isn't compatible with Win8.1
Also, noticed another oddity. I create desktop shortcuts for shutdown.exe for easy shutdown and restarts. Using "shutdown.exe /r /t 00" does NOT restart the computer. It does flash "Restarting" on the screen as it shuts down, but it never restarts. Using the Win8.1 Charm to restart does work, but not using shutdown.exe.
Avast 2014 RC3 latest version works fine in Win8.1 as does AVG 2014,as to wireless try reinstalling belkin drivers via device manager ie update drivers part and extract the files and point it,it might just need certain files only from the driver package.
I suspect my Avast and Comodo woes may have been partially due to the "upgrade" process, which usually I avoid. I prefer clean OS installs. Avast 2014 RC3 sounds like a beta?
I removed the Belkin drivers from the upgraded-to-Win8.1 system and reinstalled two different ways: From an .exe file downloaded from Belkin which installs the Win7 drivers, and also using the original DVD (CD?) which came with the device, using Device Manager to Update Driver and pointing it to the DVD. All three methods failed on the original upgrade: (a) transferred from Win8 during upgrade, (b) using Belkin .exe file, and (c) using DVD. On my second go round (reinstalling Win8, and then without modifying/customizing it or installing any programs, upgrading to Win8.1), I can install the Win7 drivers via the .exe file or using the DVD, and they appear to be accepted by Win8.1, but Windows Networking does not recognize it as a working device.