There is indeed a problem in the instructions. u-boot expects a uImage, not a zImage. power46 is u-boot-less, so its image is not a uImage. This is just a few bytes added at the start of the image, telling u-boot about its size, purpose, etc.

So indeed, of the three operations in the original post, only the middle one - replacing one u-boot with another will work. The others need an extra step of converting between uImage and zImage.

You can find a mkimage binary (for x86_64 linux) here:
http://my.svgalib.org/770/n900/u-boot/
and instructions for converting a zImage to a uImage here:
http://al.robotfuzz.com/~al/maemo/u-boot/