$ git cl upload
    Change https://chromium-review.googlesource.com/XXXX has been submitted, new uploads are not allowed
    Create new CL? [Y|n]

=> Creates new CL
- copy over original description
- add original changes as baseline patcheset
- upload new changes as second patchset


See also  http://crbug.com/gerrit/5456  which requests similar thing in PolyGerrit UI.