FIX address Stefan's comments

mne_bids/write.py

@@ -1019,7 +1019,9 @@ def write_anat(bids_root, subject, t1w, session=None, acquisition=None,
     t1w = nib.Nifti1Image(t1w.dataobj, t1w.affine)
     # XYZT_UNITS = NIFT_UNITS_MM (10 in binary or 2 in decimal)
     # seems to be the default for Nifti files
-    t1w.header['xyzt_units'] = np.array(10, dtype='uint8')
+    # https://nifti.nimh.nih.gov/nifti-1/documentation/nifti1fields/nifti1fields_pages/xyzt_units.html
+    if t1w.header['xyzt_units'] == 0:
+        t1w.header['xyzt_units'] = np.array(10, dtype='uint8')
     # Now give the NIfTI file a BIDS name and write it to the BIDS location
     t1w_basename = make_bids_basename(subject=subject, session=session,
                                       acquisition=acquisition, prefix=anat_dir,