I have a round trip Economy Class ticket, from Stockholm to São Paulo. Booking class Y on the outbound and B for the return. I've used the outbound portion of the ticket, and for my return I noticed that Lufthansa have released inventory in Premium Economy booking class E. The price for the return portion is the same as my B-fare, but when I called Lufthansa to make the change they quoted me a fare difference of $580 for booking class G. How does Lufthansa calculate the fare, e.g do they use the same fare as if the ticket would have been sold in Brazil, or fares sold from Sweden.

Is there any chance to get my ticket changed to E, which I assume is a lower booking class, or does Lufthansa prohibit fare downgrades?