Greetings,
Your system and hardware does meet windows 11 requirements.
You are correct about needing to enable Secure boot & UEFI. You cannot do this on a disk formatted MBR.
Search google for mbr2gpt
This is a command line option which can be performed in the WinRE environment. Nothing special you need to do, except to start the machine in WinRE. This is found under Settings > Recovery > Advanced Start Up.
From there, you will run some commands. mbr2gpt /validate and /convert, then make the changes to your BIOS setting for no CSM, UEFI and Secure boot.
Important I've seen this go well, and I've seen it not work. If your disk is packed to capacity and doesn't have adequate free space, the option to convert will fail. You'll see this when you run the validate option.
Also note. I strongly suggest you make a back up of any important data. If things don't work, you haven't lost anything except some time.
If you have a large disk with multiple partitions, I suggest you convert each one to GPT format. The command will only convert the boot drive unless you tell it otherwise.
The process itself is not that bad. Honestly if your install is several years old, a fresh install is preferred. Thats up to you though.
Last recommendation. Worst case, you back up your data and perform a fresh install of W11. In this scenario, you will use Shift F10 during install to convert the disk to GPT. This however will destroy all data on the disk, so once again back up your data before you begin if the conversion is not possible. As long as you have a back up, you have nothing to fear.
