As I recall, the PPC machines (like the Power Mac G5) never had EFI of any kind, and the early Intel Macs (including the Mac Pro) all had 32-but EFI even after the processors went 64-bit. I don't recall any of those Macs ever being switched from 32-bit EFI to 64-bit (U)EFI with a firmware update, or vice versa. It was a bit of a pain point because Linux was not initially ready to run a 64-bit kernel on top of 32-bit EFI, but that got resolved on the Linux side and I don't recall anything about Apple's firmware updates making that harder.