Microsoft states it is not possible to convert an existing Windows 10 installation from MBR to GPT mode. Now following how to proof them they're wrong ... :-P

• Use gparted to shrink any partition and gain at least 200 MB (better 500 MB) of free disk space. (If you already have a boot partition on your hard drive you can "recycle" it for this purpose instead and skip this step.)
  
• Create a new partition in the free space, type "ef00" (EFI). (If you already have a boot partition on your hard drive you can "recycle" it for this purpose instead and skip this step.)
  
• Format the UEFI partition with a FAT32 file system.
  
• Launch gdisk, it will convert your MBR-setup into a GPT-setup. Do this at your own risk. ;-)
  
• Boot a Windows 10 Setup Medium (DVD or USB pen drive).
  
• Press SHIFT-F10 when the initial dialog for language-selection appears.
  
• In the opening CMD enter:
  
  
  • X:\> diskpart
    
  • DISKPART> list disk
    
  • DISKPART> sel disk (your_disk_here)
    
  • DISKPART> list vol
    
  • DISKPART> sel vol (your_desired_volume_here)
    
  • DISKPART> assign letter=s
    
  • DISKPART> exit
    
  • X:\> bcdboot C:\Windows /s S: /f UEFI /v
    
  
  
• Now reboot and switch boot order to UEFI. Done!

P. S.: Yes, there is this fabulous "MBR2GPT.EXE" tool from MS which states to do the same job all in one go, even on a running OS, with only these humble restrictions (beside others):

• There are at most 3 primary partitions in the MBR partition table.
  
• The disk does not have any extended/logical partition.
  
• All partitions on the disk are of a MBR type recognized by Windows or have a mapping specified using the /map command-line option.

Choose your way.