I've had 100% success on SDC under the following circumstance, but not sure I should have...

I will fly AUH-LHR-GLA (Underlying Fare in WT+ and Economy Plus) then apply a GUF to move me into J on both the longhaul and shorthaul sectors. When I call into the call center, its a firm no for a SDC LHR-GLA. Same at check in at AUH. But when i connect through, and get to CCR, they are always happy to change the ticket to an earlier flight, and retain me in the J cabin up to GLA. Saves me quite a bit of money considering my connecting flight up to GLA is usually the last flight of the day, and the SDC is on the 6pm flight...

My understanding is a SDC is possible, but I should be moved into Y instead...

Anyways, just sharing my data point.