There's a Lot of misinformation online when it comes to MBR vs. GPT drives. Strictly speaking there's no reason to convert an MBR drive that's working just fine for you. Practically speaking, many device BIOS will automatically go into pure UEFI mode when they see a Windows GPT boot drive, though additional MBR drives are generally fine. Win11 Does require a GPT drive of at least 64GB along with UEFI boot & secure boot, but the level of enforcement may be iffy. You have the workarounds or hacks to turn off checking for hardware compliance -- Microsoft builds turning off that hardware check right into the registry. My Insider version of Win11 has been perfectly happy on an MBR drive with CSM on in the BIOS, and the official Microsoft Win11 VM comes with both Legacy booting and an MBR disk in the form of a VHD. OTOH installing Win11 fresh a month ago the setup routine tried to turn on secure boot in the BIOS, and Microsoft keeps saying how it may not provide Windows updates to anyone running Win11 on PCs/laptops that don't meet every hardware requirement. Long story short, & IMHO anyway, if you find that you've got a good opportunity to use GPT rather than MBR, e.g. migrating from a conventional drive to an SSD, you're probably better off in the long run to go with GPT. The reason for this post is that doing just that, moving Windows from an old MBR hard disk to an SSD I encountered a glitch in the process that I want to make folks aware of, though it's not likely to impact most people. You can find out about the basic differences between MBR & GPT here:
howtogeek[.]com/193669/whats-the-difference-between-gpt-and-mbr-when-partitioning-a-drive/
You can convert an *Empty* MBR drive to GPT in Windows using Control Panel -> Admin Tools -> Computer Management -> Disk Management. You can also use the Windows utility DiskPart. There's a Windows tool called MBR2GPT that will convert a drive with Windows on it, though there are several conditions that must be met. The easiest way to do the conversion is with an app like AOMEI's Partition Assistant, where you just right click a drive and select convert to GPT, but that seems to have many [all?] of the same limitations as MBR2GPT -- perhaps that's what they use under the hood.
docs.microsoft[.]com/en-us/windows/deployment/mbr-to-gpt
Now the glitch in the process I ran up against...
I had 2 copies of Windows on an old, conventional hard disk that I fear is starting to show its age. And I had an SSD that I bought on sale for less than the cost of a conventional drive to replace it. So I started out cloning the 2 Windows partitions to the SSD... the MBR/GPT stuff is part of the partition tables, so when you clone a partition or restore an image backup the target disk is going to be the same, MBR or GPT, as the original. Not a big deal, I booted to my Win2Go drive, fired up AOMEI Partition Assistant, & converted the SSD, with both copies of Windows now on it, to GPT [using the Win2Go drive I avoided Partition Assistant adding its WinPE version to the hard disk & booting to that for the conversion]. Problem is, the partition with the 1st copy of Windows on it was converted, but Not the 2nd. The 1st would boot -- the 2nd would not.
While my actual process was quite a bit longer, since I wasn't sure where or what the problem was at first, what it boiled down to was getting rid of the first copy of Windows on the original hard disk [it was already working on the SSD], & using Partition Assistant to convert that original hard disk to GPT, which worked since that 2nd copy of Windows was now the only copy of Windows on that disk. Now when I cloned that partition to the SSD, that copy of Windows worked just fine.
Getting Windows to boot on a new drive...
Moving Windows to another drive means that you'll also have to fix the BCD boot files -- they look for a copy of Windows on a certain partition on a certain drive, so you have to change things so they look at the proper partition/drive. Some Windows migration &/or tools will handle both the cloning and fixing the BCD boot files, but otherwise EasyBCD is the easiest way to fix that -- under Advanced Settings, select the copy of Windows in the upper box, set the current drive letter for that partition in the bottom box, & click Save Settings. The bootable USB sticks from backup apps like Paragon HDM or Macrium Reflect may have a boot repair function that'll work. A bootable USB stick with WinPE on it can work to run EasyBCD etc., and you can try using Windows tools to try to fix the boot files: docs.microsoft[.]com/en-us/windows/client-management/advanced-troubleshooting-boot-problems
For me the easiest route is to use my Win2Go drive, though I have been known to, in *Extreme* situations, install a fresh copy of Windows just to fix the boot files, then delete it afterward.