Q. How do I convert a Windows 10 system from BIOS to UEFI?
A. The real problem converting from BIOS to UEFI is the disk partioning is different. MBR (BIOS) vs GPT (UEFI) and there has not been an easy way to convert however Windows 10 1703 introduces the MBR2GPT.EXE tool. This is designed to be run from Windows PE however it can also be used from within a Windows 10 environment for an additional attached disk and will convert a MBR structure to GPT in addition to the creation of the EFI system partition (which is generated by converting the system partition if possible or a new partition is created) which would then enable the machine to be switched to UEFI mode (normally this is done by disabling the Compatibility Support Module (CSM)). Note that while this tool must be run from 1703 or above it can convert any previous instance of Windows 10 and possibly earlier versions of Windows (although not supported).
You can test the conversion prior to performing using the /validate switch, e.g.
mbr2gpt /validate /disk:0
Assuming all is OK convert using
mbr2gpt /convert /disk:0
Before booting to the converted disk switch the machine to UEFI.
More detail can be found at https://technet.microsoft.com/en-us/itpro/windows/deploy/mbr-to-gpt.