Yesterday I attempted to upgrade from Windows 8.0 to 8.1 on two computers.
The desktop did upgrade, but there were a few issues, and it took a long time.
The Samsung Slate ( tablet ) would not upgrade from 8.1 Preview to 8.1 Pro, nor would it revert to 8.0. Samsung has provided no support on this problem.
Others have had problems and some have had clean upgrades. If you are planning to make the switch ( which has many benefits ), plan to have plenty of time to do it, and expect that you may encounter difficulties. I would definitely suggest you do one computer at a time.
Also, there is no .ISO file for the upgrade. It is a download from the Store, and that may fail.
Good Luck.
Tim