There is a guy on badweatherbikers dot com who put an R tail on his Lightning, and did a very good job with it. His screen name on there is Glitch. He could probably give you a great deal of information on how they did it.
There are many others who have done this swap.
The problem comes in fitting all of the electronics into the R tail, because the ecm, relays, and fuses are in the front fairing on the R model (Firebolt), and are under the seat on the S model (Lightning).
IMHO it is a dam good looking swap too.