In the old 2D editor sqm, group leader did not need to be the first defined unit in a group. The group leader was defined by the "leader=1" flag.

OBS:
When importing a 2D editor mission with a group leader who is not the first defined (presumably the first placed when making the mission) unit, the resulting 3D editor mission will set the first defined unit in the group the new leader.

EXP:
The same unit should be group leader in the imported mission as in the original mission.