I tried Peters suggestion in the beginning, to be safe I tried again now, though. Here is what it says:
mareike-desktop:~/abin> which 3dvolreg
/usr/local/freesurfer/bin/3dvolreg
mareike-desktop:~/abin> which 3dvolreg.afni
/usr/local/freesurfer/bin/3dvolreg.afni
mareike-desktop:~/abin> sudo cp /usr/local/freesurfer/bin/3dvolreg /usr/local/freesurfer/bin/3dvolreg.afni
[sudo] password for mareike:
cp: ‘/usr/local/freesurfer/bin/3dvolreg’ and ‘/usr/local/freesurfer/bin/3dvolreg.afni’ are the same file
mareike-desktop:~/abin>
mareike-desktop:~/abin> sudo ls -l '/usr/local/freesurfer/bin/3dvolreg.afni'
-rwxr-xr-x 2 root root 89392 Apr 28 2013 /usr/local/freesurfer/bin/3dvolreg.afni
