Since GPT has some advantages MBR doesn't have, I would like to ask this pertinent question:
Is it possible to keep all data on the HD and upgrade from MBR to GPT? I learnt that GRUB PC can recognize GPT even though the mobo's firmware might not have an idea.

Maybe ... maybe not. GPT needs more space at the front AND at the end of the drive. If your existing partitions occupy those spaces, then the answer is no. But you might be able to move or shrink partitions a bit before the change. I always recommend making TWO backups before any change like this.

Sorry, I don't know about GRUB. I suspect only the latest version is usable for this. Linux will recognize the GPT partitions at boot or by running partprobe.

You can do almost anything to partitions - including screw them royally. See the comment above re backups. Plural.

You (probably) can't simply convert to gpt and expect everything to "just work". Grub will need a separate boot partition - plenty of article on the web, but will re-install cleanly if that exists.

It is difficult to imagine why my post should cause irritation. GPT has obvious advantages over MBR for at least two reasons which are very relevant to a situation like mine. With MBR, my data and bootability is at the mercy of the first sector, 512 bytes in all with the partition table taking just 64 bytes. These 64 bytes of data comprise an array of 4 records which determine the bounds of the four primary partitions that an MBR disk can hold. Extended partitions are nothing more than a primary partition seen as a linked list of partitions. If one link within this chain of partitions is damaged, the rest is lost! GPT neatly resolves these limitations with the added benefit of dublication at the end of the disk.

Even though the BIOS might be unaware of GPT, grub-pc can still boot because its primary stage is stored in the first sector which is not used by GPT. Since, the primary bootloader (a mere 446 bytes of executable code) only reads a series of sectors, logic dictates that booting should be unaffected.

The above two advantages are inviting me to consider upgrading from MBR to GPT.