1. Suspend Bitlocker protection
2. Start CMD with admin privileges
3. in CMD window run mbr2gpt tool to validate the disk
mbr2gpt /validate /disk:0 /allowFullOS
4. In CMD windows run mbr2gpt tool to convert the disk from Master Boot Record (MBR) to GUID Partition Table (GPT) without modifying or deleting data on the disk.
mbr2gpt /convert /disk:0 /allowFullOS
To configure BIOS for Secure Boot restart the computer and press F2 to go into BIOS Setup and change the following BIOS Settings:
General->Boot Sequence->Boot List Options->UEFI (check)
General->Advanced Boot Options->Enable Legacy options ROMs (uncheck)
Secure Boot->Secure Boot Enable->Enabled (check)
6. Resume Bitlocker
To resume BitLocker after conversion, you will need to delete the existing protectors and recreate them. (Note: My Bitlocker was set wit TPM and PIN)
In CMD window run
manage-bde -protectors -delete c:
manage-bde -protectors -add c: -TPMandPIN -rp
manage-bde -protectors c: -enable
If there is an error message when turning bitlocker on rename the %WinDir%\System32\Recovery\ReAgent.xml file to something like ReAgent.xml.old and try it again.