I've had mixed success making SDC with confirmed SWU's.
Past data points:
1) I had confirmed SWU on JFK-LHR. Had to call couple times for agent to change me to an earlier flight on the same day(I inventory, C was not available at the time of change)
2) I had confirmed SWU on JFK-LHR. Called 5 times and every agent said that there is no inventory(E). They said its an economy ticket and need E inventory to move me to earlier flight in Business. Made no sense.
What is the correct policy? Do we need E inventory for premium SDC? Do we need C space when using confirmed SWU?
I will be trying to change from 104(SWU confirmed) to 106 next week.