Uh, mhulshult that thread isn't helpful to the question that was asked.
When you get a guy in trade, and he has options remaining, you can assign him DIRECTLY to AAA (or wherever) without a penalty. I've done it 10+ times over the seasons and never once had a "demotion penalty".*
If you put him on the BL roster and then demote him, you may be subject to a demotion penalty.
ETA: I'm not saying it's not possible, I'm saying I've done it more than 10 times and I've never seen it. Your results may vary. But post an example.