As mentioned in that thread, you don't really convert between the partition schemes.
That thread is about a GPT partitioned disk that was then reformattted using MBR leaving the backup GPT table at the end of the disk where it's detected by disk tools. The "fix" is to delete the backup GPT and just use the existing MBR. Or to reformat the disk using GPT.
Either way, the data on the disk may not be retrievable.
