When I SDC, I forego certain things (or entitlements one may say). First is the ability to get an upgrade. I'm not taking my original flight, which I took a spot in the upgrade list away from someone else. I chose not to use it. Second is that I don't have an issue with being stuck in the middle seat if that's the only seat available. Bummed out and cramped, probably, but if it gets me where I intended to go at the time I prefer vs. the original, well no complaints from me.
I look at it this way, I'm already probably bumping some other lower elite or general member one down the list just trying to get a seat. I shouldn't expect to trump others just because I changed my mind.
That's independent of how Inventory Management deals with giving out the upgrades in advance. They do their own sort of voodoo calculations but my principles on SDC upgrades remain the same.