One of those bits of legacy nonsense still with us is the old FDISK based partitioning scheme, which still has nonsense like cylinders, heads and sectors in it (which mean nothing at all to modern disks, which in essence are just single-dimensioned arrays of 512 byte blocks).
FreeBSD has existed within this framework by using the old BSD "slice" system to chop up a single FDISK partition. Linux, instead, has tended to use FDISK extended partitioning, but that has led, to some extent, to Linux systems living in a single filesystem. One of the benefits of the multi-partitioning that FreeBSD uses instead are that two of the typical partitions (/ and /usr) tend to be "read-mostly," while the other two (/var and /home) tend to have the majority of write activity. This is somewhat safer, since the volatility is kept away from important configuration data (in /etc) and system code (in /boot and /usr).
Macs have discarded a lot of the Intel legacy, since they don't have to boot 20 year old operating systems (instead, they boot Unix, which is a 30 year old operating system, but I digress). So the Intel macs have adopted GPT partitioning. GPT is great, since it can operate on disks with 2^64 blocks, which is ridiculously huge, and the number of partitions is unlimited (it's actually limited to the number of blocks allocated to the GPT itself, and by default it's 128 entries).
Well, with the advent of FreeBSD 7.1, support has been added to boot from a GPT partitioned disk. The way this works is that the legacy FDISK partition (which basically marks the entire disk as allocated to protect against FDISK based tools trying to do something to the disk) contains boot code that looks for a special GPT partition that contains the second stage boot loader. That partition and the FDISK boot sector is written with the 'gpt boot' command. That boot loader looks for the first UFS partition on the disk and loads the main FORTH-based loader, which in turn loads the kernel and so on.
Just to be on the safe side, I migrated my setup by buying a new disk (a 320GB disk for $55 - can't go wrong!), partitioning it and then copying all the stuff over. As a bonus, the new disk is faster than the old one, and we get to start the warranty over.
There is, however, a 'gpt migrate' command that purports to convert a disk from FDISK and BSD slice format to GPT, but that concept sounds a bit scary to me, at least for filesystems I really care about. :)
One interesting thing is that the gpt command opens the disk for writing, even if all you want to do is read information. But if there are mounted partitions, then this will fail. There is a -r option that will make the open read-only, which is useful for the 'show' command.
Now that the disk is GPT partitioned, converting to an EFI based machine would simply require replacing the special FreeBSD boot partition with an EFI system partition (it would need to be slightly larger, but space from the adjacent swap partition could be stolen), and EFI boot code for FreeBSD would need to be added to it (if it existed).