One of my first questions to the forum was whether this was possible without structural alterations, and while it's probably not of interest to many it'll make a big difference to me.
Good news for me, it was as easy as I'd hoped. Simply flip the shift arm on the selector shaft upside down and 'hey presto', race change.

I think for perfect operation the arm could do with being nearer to vertical which will require a shortening of the link bar but that's for another day.