Be careful when you say "cheapest fare." Saver fares are not eligible for SDC Full stop!
I too am not 100% sure on how upgrade requests are processed on a SDC. It would appear that I was "slotted" into the upgrade waitlist having purchased an eligible fare for GGU application. I was in position #4 of 11 after my SDC about 21 hours before departure. #1 on the list cleared at 2:00 am, 13 hours before departure. The rest of the upgrades processed at ~T-2 hours.The waitlist remained at 11 yet after my upgrade, I was in position #2 .
James