Several users have found they can migrate from 5.0.8 CSM to 5.1.0 UEFI simply by booting the installer/upgrader in UEFI mode. While this works, it isn't really supported and fails to copy in several files (e.g. the new boot logo). Similarly, switching an existing 5.1.x CSM install to UEFI isn't supported either. To fix this, I wrote csm2uefi.exe (attached).
To use it, you start with an existing 5.1.x installation (CSM or UEFI), mount the 5.1.x ISO (doubleclick on the file) or your 5.1.x DVD, then run:
csm2uefi <isoDrive:> <5.1bootDrive:> [e.g. csm2uefi F: C:]
It will copy in any missing files, then update 2 video-related ines in 'config.sys' to make it ready to boot in UEFI mode (the original is saved as 'config.csm' if you need to revert). If you're doing a 5.0.x to 5.1.0 upgrade, I'd suggest doing it in CSM mode so you can confirm the upgrade went as planned. Once you're confident everything works, then do the CSM->UEFI migration.
Important
* this does not create or setup an EFI System Partition - use the options on MiniLVM's 'System' menu when booted under UEFI to do so.
* the ISO and the installed system must be the exact same version of AOS - this method can't be used to boot other versions of OS/2 under UEFI (there's a different method available but that's another post...)
Update
* v2 - fixed a bug in v1 that caused a failure when calling 'CUBE'; v2 supports both 'cube.cmd' and 'cube.exe'