This happened to me two weeks ago. I was upgraded into NC on SWUs from BKK-NRT-SFO. BKK-NRT portion was canceled. There was space available in C on SQ from HKG-SFO, but gate agent refused to re-book me into C because I only paid for a Y ticket (Q class). I pleaded, but in the end, they re-routed me on TG into HKG and then CX into SFO, all in Y. Being 1K didn't amount to much other than I guess it got me home that day. Someone else on my flight, a 1P, said they were re-routed on TG into DXB in C (poster had paid for Y but used SWUs to upgrade) and then C from DXB-IAD. That was four hours after my wife and I had left BKK (as soon as GA announced unspecified "mechanical" delay, I went to the desk and asked to be re-routed ASAP).