I just put the Windows 10 ISO from the Microsoft site on a USB, kicked the install off, entered the Windows 8.1 licence key from the OEM sticker on the machine and it happily installed and activated.
Not sure if I'm an edge case with some peculiarity with our hardware vendor, but was surprised to learn the upgrade window finished sometime ago...
If you've made any of the machines, e.g. by buying a motherboard or a barebone and then by installing Windows 8.1 yourself, it's a news to me.
EDIT: I'm especially interested in upgrades, not in "clean" installs. Most of the users don't want to start from the scratch with configuring every program they use.
OEM BIOS unlocks tend to not work between Windows versions. I've experimented with this a little. My go-to method to upgrade these is to clean install the version that will BIOS unlock, activate it, then upgrade that to Windows 10. After you upgrade, you can clean install Windows 10 on the system and it will activate normally.