Upgrade MBR to GPT without losing data
Date Added: 11/26/2019

Windows 10 will not allow you to upgrade past v1803 if your disk is formatted as MBR.  You must upgrade it to GPT.  Normally this involves wiping the disk and reinstalling everything from scratch.  But this is how to do it without losing any data or reinstalling anything:

Do not attempt this unless you understand everything you're doing or else you could lose all your data.

1) Make a full image backup to an external drive.  I used the built-in Windows System Image Backup tool.
2) Verify your backup is good, then unplug the external drive for now.
3) Boot from a Windows installation USB and delete all the partitions from the target disk.
4) Allow Windows to create all new partitions on the target disk.  It should create about 4 of them, one of them will be a "hidden" 100MB FAT32 EFI partition.
5) Once the installer gets to the next screen and begins copying files, hold the power button to power off the computer.  (You could wait for it to completely finish installing Windows but that's a waste of time since we'll be deleting that partition soon)
6) Boot from Partition Magic or some other partition management tool.
7) Delete the newly created C: partition from the target disk, leaving that area as unpartitioned space.
8) Plug in your external drive that has the image backup on it.
9) Mount the image backup's VHD file.  With Partition Magic, I had to minimize Partition Magic to see the boot options and select the option to open a command prompt, then I used diskpart.exe to mount the vhd file using the vdisk features of diskpart.
10) Now that the backup image's disk and partitions are visible in the Partition Magic program, copy the backup's C: to the target disk's unpartitioned space (where you just recently deleted the temporary C:).
11) Now that the C: partition from the backup is copied onto the target disk, shut down the computer and unplug the external drive.
12) Boot from a Windows installer USB again and go to the repair menu's command prompt.
13) Follow the instructions on my other Knowledgebase article titled "bootrec fixboot - access is denied (GPT disks)" to erase and recreate the EFI partition.

Once the EFI partition is repaired using that article, Windows should boot normally.  It will still be activated, you can install the latest Windows Updates, and everything should work great without any data loss.

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