Having completed the process I can update on the process, which was pretty straightforward.
Firstly I did my backups. Copied data, imaged with Macrium, created recovery USBs with built in Recovery Media Creator, and backup up drivers separately with DISM.
First run cmd with administrator privileges
Verify drive for conversion:
mbr2gpt /validate /disk:1 /allowFullOS
Disk layout validation failed for disk 1
Failed due to too many partitions. It looks like I have two recovery partitions. Check active recovery partition:
reagentc /info
Windows RE location: \\?\GLOBALROOT\device\harddisk1\partition3\Recovery\WindowsREBoot Configuration Data
Delete other recovery partition:
diskpart
list disk
Disk ### Status Size Free Dyn Gpt
-------- ------------- ------- ------- --- ---
Disk 0 Online 931 GB 0 B
Disk 1 Online 256 GB 0 B
select disk 1
list partition
Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 Primary 500 MB 1024 KB
Partition 2 Primary 254 GB 501 MB
Partition 3 Recovery 562 MB 255 GB <-- partition 3 is active
Partition 4 Recovery 509 MB 255 GB <-- partition 4 is inactive
select partition 4
delete partition
Cannot delete a protected partition without the force protected parameter set.
delete partition override
DiskPart successfully deleted the selected partition.
Attempt validation again:
mbr2gpt /validate /disk:1 /allowFullOS
MBR2GPT: Validation completed successfully
Reboot system into advanced startup and run command prompt.
mbr2gpt /convert
[...]
Call WinReReapir to repair WinRE
MBR2GPT: Failed to update ReAgent.xml, please try to manually disable and enable WinRE.
MBR2GPT: Before the new system can boot properly you need to switch the firmware to boot to UEFI!
Close > Turn off your PC
System booted fine regardless of the MBR2GPT final comments.
I enabled secure boot and fast boot.
I did end up doing a fresh install, the above was just to test the MBR2GPT tool.
I couldn't do a fresh install onto the new SSD with the created recovery drive (kept failing), so I ended up creating an installation flash drive with the downloadable Media Creation Tool, and using that.
Also, after all this, I put in the second SSD, and it didn't show up. I re-seated it, and enabled secure boot (I'd disabled it at some point), and it showed up. I really didn't expect seating to be an issue, because the drives slide in pretty tightly and are screwed in place. But I also really wouldn't expect any BIOS settings to make a different.
Anyway, it all seemed to come out fine.