XGEN BLENDSHAPE TOOL
This tool allows transferring XGen from one geo to another. You will need:
  1. The original base mesh (Old Base)
  2. The new base mesh that you want to transfer XGen guide description over to (New Base)
  3. The scalp mesh (Gro Mesh)
  4. The Old Base and New Base meshes must have the same topology.
If you do not have a separate scalp mesh, you will have to duplicate the "Gro Mesh" geometry and use that as the "Old Base" instead.

Let’s say we want to transfer from the Old base to the New Base.
1. Select your growth mesh / scalp and set it to the GroMesh slot. You can do multiple ones if they were extracted from the same body base geo and are in the same Xgen collection.
2. Put old base to Old Base slot and new base to New Base slot
3. Click Blend, Xgen will now transfer over to the new base.
4. ​You will see the old base is now blendshapped to the new base. Unhiding Xgen will give me this result. (The Xgen Guides will ‘rotate’ base off the face normals’, as my new base has a lump on his head, the guides will rotate up. This is normal behavior).
5. IMPORTANT: click on ‘Clean Up’ To finalize Xgen. This step is quite important. It needs to be done every time you do a blendshape to clean out the unnecessary nodes in the back end.
6. We just have one more step to do. If you go into any of the Clumping modifiers and click on ‘guide’ or ‘generate’ button in the setup maps, you will see that the point display is actually still in the old position. 
7. ​Simply Save your Maya scene and reopen it and you will see the clump points are correct now.