It has always worked for me in the past as well, and I have done it several times The agent this morning tried to upfare into G, was blocked by the computer, put me on hold a long time while she tried to clear the upfare and then informed me that there was now a $200 per ticket change fee unless I was upfaring into first class, in which case there would be no change fee.
If there are recent data points to the contrary, I would be very interested in them.