I've upgraded a bunch of unsupported Windows 10 PCs to 11 successfully.
Some of them were Legacy boot. And they booted fine after I upgraded.
One did not and I destroyed it trying to fix it.
I think I could have gone to BIOS and bumped the bootloader to #1.
In any case, I should have done this before the upgrade.
Check to see if UEFI is available in BIOS
then
CMD admin
mbr2gpt /validate /disk:0 /allowFullOS
if passes
mbr2gpt /convert /disk:0 /allowFullOS
then
Go to BIOS and select UEFI boot
If I doesn't have an EUFI BIOS, don't convert to GPT
Summary of what I think
Windows 11 will boot on Legacy only bios
A UEFI BIOS may be set to boot Legacy but it is a UEFI bios
A Legacy only, non UEFI bios can not handle GPT. It's MBR only