Full Idea
The word 'pain' is a non-rigid designator; it is a contingent matter what state the concept and the word apply to. (Note: so the sort of theory Kripke argues against is not what we propose).
Clarification
'Rigid' designation applies in all possible worlds
Gist of Idea
The application of 'pain' to physical states is non-rigid and contingent
Source
David Lewis (Mad Pain and Martian Pain [1980], §III)
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.125
A Reaction
I like the view that a given quale is necessarily identical to a given mental state, but that many mental states might occupy a given behavioural role. The smell of roses might occupy the behavioural role of pain. Frog pain isn't quite like ours.