At one time it was possible for a diamond to lose an upgrade to a silver. If the silver was on an earlier flight that went into irops or they took an IMDb voucher sometimes the agent will decide the ticket as a full y. At one time that would hve trumped any diamonds in the upgrade list unless they were also on a y fare. It's my understanding delta but a stop to that nonsense recently and the y fare only trumps others in your same elite level and. It higher tier elites