Thank you for this. I will try it out.sypralout... I'm attempting to do the same thing. I think you have done it in the wrong order.
Assuming you have a customized parent that you want to upgrade
1) Export the customized parent style as an archive (I think this will drag along pictures, etc., that are not in the xml).
2) Upload the customized style as a child of the parent. Afterwards click on it to edit it's description to identify it as changed or customized.
3) Set the child as the default and user selectable.
4) Install the updated version of shades of grey as the parent, replacing the original. (The old child will stay with your customizations)
(This is just what I got from reading stuff... I'm waiting till my users are absent to do the actual upgrade. I'll delete this line if it works.)