I did a test update on my T420 and that worked great, but the installer doesn't switch the disk to GPT and doesn't add an EFI boot partition, but installs the EFI drivers.
The biggest problem here is invalid assumptions and expectations. The first is that the installer was designed to upgrade an existing partition from CSM to EFI. It wasn't: it was intended to upgrade 5.0 CSM to 5.1 CSM _or_ a do new 5.1 EFI install. As you have demonstrated, the CSM->EFI upgrade path is available - you just have to finish the job (see below).
The other incorrect assumption is that "something" would automatically convert your disk to GPT even though such a feature isn't mentioned in the documentation nor does it appear in the installer's UI.. As it happens, the only known OS/2-based MBR to GPT converter is sitting here on my hard drive. I offered it to one of AN's principals but he declined the offer because he couldn't imagine a reason someone would want such a thing. Sad but true...
To finish your CSM->EFI conversion, you just need to create the EFI partition. Here's how:
* boot up yur new install using the USB strick
* open MiniLVM
* select System->Create EFI partition; follow the instructions
* select System->Boot Menu->Install/Update; follow the instructions
* eject the USB stick and reboot - you should get the AN Launcher menu (i.e. AN's EFI boot manager)
Edit: MiniLVM offers these options for a GPT disk. I _assume_ they're also available for MBR disks. If not, it's still easy to create a bootable EFI partition - you just have to copy some files.