you can mv or copy the whole /usr to another partition, edit the fstab correctly, and even without rebooting, just with a mount -a, your done! (i did this yesterday) that is one of the advantages of the linux file system. It doesnt even have to be a partition, you can mount it under a subdirectory, etc... I wouldnt do that with / so easily though ;D i have never tried, but that would mess up grub and lilo config and i dont know what else...
but you have to be very careful when you move or copy, that all the attributes (ownership, priveleges. etc..) are conserved. Keep a boot disk or a knoppix cd nearby just in case anything goes wrong.
i copied (man copy for details on how to conserve attr.) and then used kdiff3 to check that absolutely all files where identical...