I remember when doing mbr2gpt, I did that first, then restarted, went into BIOS and my Asus board had already recognized it as UEFI Windows boot manager, using a 950 pro NVME in the m.2 slot

This was a x99 strix.

Samsung 970 EVO NVMe M.2 drive plugged into an M.2 socket on motherboard.

Reason I ask about all this is because I am setting up WIndows 10 on another system and switching back between a Windows 7 non UEFI drive and a Windows 10 UEFI drive proved problematic with much configuration changes in the BIOS necessary to get the system to actually boot.

Also, did you use the AllowFullOS switch to convert from within the running environment?

My Z390-A also switched to internal graphics when something in the drive menu wasn't set to Legacy mode, and I spent hours last month trying to figure out why I had no video display after updating the BIOS. Two things had happened: the drive type defaulted to whatever is the non-legacy mode (so it couldn't find a boot drive, and when that happens, it forces the the internal video and ignores the PCI video card. I eventually had the idea to try plugging in a displayport monitor on the motherboard video and saw the boot process there. Spent an hour trying to figure out why it would not let me select PCI graphics. Then I remembered that the disk feature used to be set to Legacy (something which bothered me, because the NVMe drive is pretty new tech--I didn't realize Legacy meant MBR format) and I went and set that to Legacy and then the board would allow PCI graphics again.

I'm just concerned that I don't get into that mess again during this conversion. I spent a month tweaking and working out the quirks in Windows 10 Pro, getting my legacy scanners, tablet and MIDI stuff all working and configured.. 100+ hours of troubleshooting, research, driver hunting, hacking and modifying config files to get everything working. Would have to lose it all!

I plan to do this over the weekend.

Is there any danger to using the AllowFullOS switch on the MBR2GPT command to effect the conversion from within the Windows environment, or should I boot from a recovery environment to do this?

I successfully converted my MBR boot disk (an M.2 NVMe drive) to GPT and booted into UEFI and turned off Legacy mode and verified proper Windows boot loader. Booted up into Windows 10. The odd thing is it doesn't turn on the monitor until I press a key, where I then see the log on screen.

I was used to AutoAdminLogon in Windows 7, so I just used netplwiz and set my account to not require PW to log in.

Not my system boots in 10 seconds to the desktop from hitting the power button. I think I can shave off 2 more seconds by eliminating the delay of 2 seconds to display the boot prompt (now pointless in UEFI protected mode).

There are probably other tricks to improve boot time. I know Linus Tech Tips got their system down to under 5 seconds boot by doing some extreme measures.

Thankfully all my legacy hardware, such as slide scanner and MIDI interface still works.