I take your point, but courses are being re-rated all the time; all courses are supposed to be re-rated at least every ten years, and new courses within 5. I'm not sure how practical it would be to be adjusting masses of scores for all golfers who had played ANY course that had been re-rated, even in the current calendar year.
And since your index is based now only on the 8 best of the last 20 rounds, the problem should take care of itself pretty quickly for active golfers, especially this time of year.