This helps to explain why DL always downgrades FC rewards tickets (purchased FC ones, not upgraded) first, regardless of the status of the passenger. That way, the only compensation they need to give is in Skypesos, which they don't really care about, not in cash.
I don't think this is the case. We had a discussion about this a couple of years ago, probably pre-merger, and a DL insider (not sure if it was THE DLinsider) stated just the opposite. I'll try to find the post, but the gist was that a purchased Y ticket upgraded with miles would be downgraded before a full award F ticket.