How do I convert an initialized disk back to a raw disk?

Question

I tried to initialize an mbr disk drive to gpt.
To do this, I tried to clear-disk and then initialize-disk using powershell commands.
but I was not able to convert it to gpt because it was already initialized.
So I was wondering...
Is there any way to turn a disk that has already been initialized into a raw disk?
Also, how can I change the disk from mbr to gpt?