in my limited experience (domestically) i have been able to do a SDC including a different routing for the $ difference in fare class and w/o a change fee, which in some cases is actually a very limited cost differential (tens of dollars). this has been within Y only but if you are applying GPU and R is open on the flight you are changing to, I have to imagine that doing the SDC then applying GPU shouldn't change the outcome. now, the probability of R being open on day of departure is nil, but that is neither here nor there.