Nobody knows for sure -- probably including the employees -- because the policy they published in the wake of the Dao incident was contradictory. However, to the extent that the policy didn't contract itself, the lowest-fared Y passenger should be first to be IBDed. (This also limits UA's required compensation). And while I could definitely see an employee taking the path of least resistance and IDB'ing the J passenger, that's only going to happen if Y is full; if J is oversold, UA may well have capped the number of seats they'll sell in Y to account for it. Plus, IDBs are only going to happen if nobody volunteers -- and they will absolutely accept a Y volunteer and then downgrade a J passenger if necessary.