There are many UEFI-capable devices that are running Microsoft Windows in BIOS emulation mode today. This is happening because these systems are running older versions of Microsoft Windows or for application compatibility reasons.
Windows 10 security features including Secure Boot, Device Guard and Credential Guard require UEFI firmware. Upgrading devices from Windows 7 and Windows 8.1 in BIOS mode to Windows 10 is not enough to enable these features.
Prior to the Windows 10 Creators Update, in order to enable UEFI this would require a wipe and reinstall. This is because Microsoft Windows requires GUID Partition Table (GPT) disk partitioning when booting in UEFI mode, where as Master Boot Record (MBR) is required when booting in BIOS mode. Previously there is no Microsoft-supported solution to perform a non-destructive conversion to GPT.
With the Windows 10 Creators Update, we are releasing a tool called MBR2GPT. It supports in place conversion of disks from MBR partition to GPT. This does not modify or remove data that is already on the hard disk drive. It simplifies deployment by removing the need to back up and migrate data to an external location. It configures system partition for UEFI boot and preserves drive letter assignments.
MBR2GPT does not support configuring firmware mode. Switching firmware to UEFI mode typically requires manual configuration in the firmware menu. Secure Boot enablement and Compatibility Support Module (CSM) disablement are mutually exclusive and may need to be configured in addition to boot modes. OEM's can provide a tool to switch firmware mode programmatically.
MBR2GPT can be used offline or online. The tool is included in both of the Windows Pre-installation Environment (PE) as well as the full Windows environment. Running the tool from Windows PE is recommended to avoid potential interference from other software. The tool must run in an environment based on the Windows 10 created Update but can convert disks with previous versions of Windows 10 installed. It can be run in validation-only mode. This way it can verify the selected disk can be converted without actually performing the conversion.
For more information on MBR2GPT, please see the MBR2GPT.EXE article on Microsoft Technet.