Why?
You have spent a lot of time modifying your body shape then you buy a new head, but your body shape deforms your new head, you must use a new shape and adjust the body parameters to the new shape, you can do it manually and spend your time, or export the shapes and modify the XML files by yourself.
Why not use a tool make it for you in a few seconds? And the best is you don't have to download any program, you can do it online.
How?
1.- Export the head and body shapes:
Wear your head shape and export it as XML file. Now wear your body shape and repeat the same process.
2.- Use the Second Life XML Shape Merger
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3.- Go to Second Life, create a new shape, edit it, click on the button Import and select the saved XML file.