As Responders, we're going to have be very careful with advising changing O/Ses as no one person knows all the manufacturers EFI Bioses and their settings.
Builder machines are NO problem but OEM machines must comply with MS specifications for Windows 8 machines so some might not have the necessary settings in the EFI Bios for changing O/Ses.
For instance I just tried an experiment with a Builder Machine in which I found I could effectively NAIL Windows 8 to the machine and lock out all other ways to boot.
Fortunately my Asus UEFI Bios would let me change [ numerous settings ] so again I could boot from non-secure devices and also run Windows 8 on an MBR Array INSTREAD of a GPT Partitioned Disk.
Be careful for what you wish for concerning security as I can see a lot problems coming down the road with OEM Machines sold to run Windows 8 where the O/S is NAILED to the machine EFI Bios and can not can be changed.
Buyer Be Ware for sure !!