You can view your mesh not deformed by bones by unchecking Deformed in Edit mesh mode.
Meshes - Spine User Guide: Deformed
Unfortunately, I just tested it, and you cannot rotate the vertices, only translate them to a new position.
What you can do is manually reposition each vertex instead (with Deformed checked so it doesn't wreck the mesh)
Therefore, I think your best bet is this:
- rotate the new mesh (which could be a copy of the old one with the new artwork inside it) so it matches very precisely the old mesh
- select one or more vertices from the old mesh and press
CTRL+C in world axis
- select the corresponding vertices in new mesh (in the same order if you're copying the same ones) and press
CTRL+V
This should be reasonably fast and painless to do, but still, a hassle if you have many vertices. At least this way you shouldn't need to redo manually the weights :>
