SDC after using RPU

I've got an upcoming PDX-SFO where I have applied an RPU to a flight that was originally R0 but it has since cleared, several days out. (and yes, the RPU was expiring, I would never use it on that route otherwise :p )

Say I'm on flight A, which is currently F2 R0 Y9 S9 (I originally booked S class).

On the same day, there are flights B, C, and D, which have the following loads:

B: F0 R0 Y9 S5
C: Y5 S0 (no F cabin)
D: F9 R9 Y0 S0

Assuming the loads don't change until T-24, I believe that I could do a same day confirmed change to flight B, into the Y cabin, and then waitlist for upgrade. I don't think I could do SDC into flight C, right?

But how about flight D? Since I'm already "booked" into R, if R is still available, can I SDC onto that flight at T-24?

Thanks.