Changing from BIOS/legacy mode to UEFI mode

The main blocker, especially for releases up to 1607, is installations done in BIOS or legacy mode. You can perform an in-place upgrade in BIOS mode, but you will end up in BIOS mode again. A lot of enterprise customers want the benefits of the new UEFI mode, especially the new security features, which require the secure boot feature, which itself requires UEFI mode. So you end up having to decide between doing a smooth and fast in-place upgrade and not being able to use UEFI, secure boot and dependent services such as Credential Guard and Device Guard, or you can perform a classic wipe and load with all its problems of losing settings, needing to store user data, and so on.

Beginning with release 1703, there is now a method supported by Microsoft to change from BIOS mode to UEFI mode without installing an OS. The required tool, MBR2GPT.exe, is now included in the OS sources. MBR2GPT needs to be executed in offline mode, so run it from within Windows PE or attach the disk as a data disk to a running OS and execute it from there. If you are already on release 1703 or higher, you can convert it before upgrading to a new OS. If you are on a lower release, you first need to upgrade to 1703 at least. The tool can only prepare the disk, convert the MBR to GPT, and update the boot entry. To change the firmware itself from BIOS to UEFI mode, you need to use the manufacturer tools or do it manually.