It is easy as pie!

Prerequisites:
– Start the computer in BIOS-mode (not UEFI!)
– Take a full backup of your machine, or two. You can never have too many backups :)
– Have at least a 300 MB partition/free space before your Windows partition on the hard drive. The default Windows installation does this for you!
– Have a Windows 8.1 Installation USB/DVD available.
– Download gptgen.

Let’s get started!

– Run gptgen.exe -w \\.\physicaldrive0 in an elevated command prompt. After running this your computer will not boot in BIOS mode any more
– Restart your computer from the Windows 8.1 Installation USB/DVD, click into “Repair your computer” -> “Troubleshoot” -> “Advanced Options” -> “Command Prompt”.
– Run diskpart.
– List all your disks with list disk.
– Select the disk you want to convert from BIOS to UEFI with select disk 0.
– List all partitions on the disk with list part.
– Select the 300 MB partition with select part 1.
– Delete the partition (if it’s not already free space) with delete part.
– Create the EFI partitions with create part EFI size=100 offset=1.
– Format the EFI partition to FAT32 with format fs=fat32 label=”System” quick.
– Assign a drive letter with assign letter=S.
– Create the System partition with create part msr size=128 offset=103424.
– Select your Windows partition with select part 2.
– Assign a drive letter with assign letter=C, this will be used later.
– Now we need to copy the UEFI files from the Windows installation to the EFI-partition.
– Exit diskpart with by typing exit.
– Run the command bcdboot C:\Windows /s S: /f UEFI.
– Restart your computer again, go into BIOS/UEFI and enable UEFI, and enjoy!