As I understand it... Each student is required to have an ecclesiastical endorsement from one who is considered by the student to be their ecclesiastical leader. If the student leaves the congregation of that leader and goes to another. It is the responsibility of that leader to revoke the endorsement as it is to be provided by the student's current leader who can attest to the one who he or she endorses. If the student does not have a person who they acknowledge as an ecclesiastical leader it will be hard for them to obtain an endorsement and would be disqualified from attending BYU.

If the ecclesiastical endorsement of ANY student is revoked the student MUST provide a new endorsement. Essentially the same thing would happen if A member moves into another ward. The student MUST obtain an endorsement from the new Bishop of the ward they move to. If a new endorsement is not obtained the student (Member or Non-Member) would be excused from BYU.

The issue here is timing. If a student (Member or Non-Member) removes themselves from the religion whose representative endorsed them before they find another ecclesiastical leader to endorse them, they will find themselves without an endorsement and by default are disqualified from attending BYU.