I'm just wondering if there would be any problems with just using object mirroring on X(In object mode, ctrl+m->X->left click) instead of having to run the python script on the lattice to swap the 01, 03?
Unfortunately Lattices features are still lacking Dwayne.
No mirror modifier exists for Lattices - and if you scale the x by -1 (which is what Ctrl M does)
it affects the deformation and incorrect results even if you apply the deforms.
So at this stage the best option is to use python to fix things.
One day my C++ skills will be at a level I can fix stuff like that, but at the moment I can only fix basic stuff.
(and have my first actually real code accepted into the upcoming 5.0 release)
Using Blender 4.5.5 I was able mirror the Lattice on it's Global X. Change the X location to negative. Run Wayne's script to change the vertex groups from .L to .R, with the [Swap _01 and _03] portion commented out.