Everyone joins as an S/O, from there what you do is entirely up to you, with the restriction that you must be a 2 engine F/O before being a 4 engine F/O, and you must be a 2 engine Captain before being a 4 engine Captain.
Every other scenario of movement is allowed, provided you have the seniority to make the move, and provided the move is 'up' the foodchain (which essentially means you must move to either a bigger aeroplane in the same rank, or up in rank).