If you want to convert a legacy boot to EFI boot - the system disk must be partitioned using GPT.
If it is not - you will have to repartition and format the disk.
While the gpart utility can convert MBR to GPT - the likelyhood of dataloss is possible - thus not recommened unless you have safeguards in place - something like either backing up valuable files or a diskimage using clonezilla (because clonezilla images are smaller than dd’ed images)