Reassignment of I-485 to alternate eligibility

In a recent meeting, Nebraska Service Center has clarified why, some times, in cases of pending I-485 applications, USCIS approves AOS interchanging the applications of the derivative and principal applicant:

It is critical to note that NSC cannot assign a priority date from one person to another, but the I- 485s can be amended if there is alternate eligibility. The priority dates assigned to one individual can be moved between classifications within the parameters of the Pearson Memo. If current principal husband has an approved I-140 (EB-2) with a more recent priority date and the derivative wife has an approved I-140 (EB-3) with and earlier priority date, you cannot switch priority dates and assign the earlier date to the husband. However, if the EB-3 for the derivative wife is current, the husband's I-485 can be re-classified as the derivative and the wife's as principal, to permit the approval of the applications.