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.
So, I can boot via CSM or EFI (via the USB stick) and in EFI mode Win/OS2 will start.
I didn't test yet to convert the disk with Linux (GParted, DFSEE wants to delete all your partitions for the conversion) and add the EFI partition to it.