I clicked on my motherboard's page and under the BIOS tab of the page is where I found the UEFI Upgrade so I would think that it is supported.
That's the first time I've seen an available BIOS upgrade from legacy to UEFI. Let me/us know what you find out from Gigabyte.
If yours won't upgrade, it may be due to the motherboard version you have. (Although, I would expect that to be stated as a disclaimer)