The default staff-relative placement of beams is determined by the staff positions of the notes within the beamed group and their resulting stem directions.
This means that the note furthest from the middle line of the staff determines the placement of the beam, although there are exceptions to this rule and other considerations that can influence the staff-relative placement of beams.
Changing the staff-relative placement of beams involves changing the direction of the stems in the beam. Therefore, Dorico Pro categorizes changing the staff-relative placement of beams as a stem change.