if I want to switch my current install of 5.0.8 CSM to 5.1 UEFI [...] could I go from CSM to UEFI via the upgrade path? [...] I think I could accomplish this via an upgrade and then follow the steps as Rich posted to finish the CSM to UEFI conversion.
FWIW... I don't have the final 5.1GA yet _and_ I don't have a dual CSM/UEFI machine any more, so I can't test most of this stuff the way I normally would. However, after doing 100+ UEFI installs, many with non-standard configurations, I think I've gotten the hang of it...
If the installer will let you do an upgrade when booted in UEFI mode then you should end up with all the files needed to boot either way. The only difference between a fully-functional CSM system and a UEFI system are these two lines in 'config.sys':
[CSM]
DEVICE=C:\OS2\MDOS\VSVGA.SYS
SET VIO_SVGA=DEVICE(BVHVGA,BVHSVGA)
[EFI]
DEVICE=C:\OS2\MDOS\VEFI.SYS
SET VIO_SVGA=DEVICE(BVHEFI)
So, if you run into problems getting UEFI going, you can update config.sys accordingly then revert to CSM.
Be aware that if you don't already have an ESP (EFI System Partition), creating one on an MBR drive has some gotcha's. First, it must be a primary partition - 256MB is typical but an OS/2-only system can get by with ~8MB (1 cylinder). More important, when scanning an MBR disk, most systems will examine each FAT32 partition looking for '\EFI\BOOT\BOOTX64.EFI' and will ignore any other files. MiniLVM's install routine accomodates this by putting 2 copies of 'launcher.efi' in '\EFI\BOOT', one of which is renamed BOOTX64.EFI.