You can try gptgen.
Gptgen is a tool to non-destructively convert hard disks partitioned in the common, "MSDOS-style" MBR scheme (including extended partitions) to use a GUID partition table (GPT).
Usage: (on Windows)
gptgen.exe -w \\.\physicaldriveX
X is your disk number. If you don't know your disk number, you can use the command:
diskpart DISKPART> list disk
After "gptgen.exe -w \\.\physicaldriveX"
, you must restart your PC.
As for the risk of loosing data, this kind of operation should always be preceded by a full backup.