I ran 3drefit -xdel (and -ydel and -zdel) on a .nii, and it threw no error, but 3dinfo indicates nothing was changed. After using 3dcalc to make the .nii into a HEAD/BRIK pair, the same command changes the voxel dimension without a problem. I'm not asking for it to be resolved (since I can just flip back and forth from NIFTI to BRIKs, and vv), but thought it should be posted so other people could find it.
Thanks!
-Daniel