There are two ways I would suggest:
1) Use "Insert Before Selected" in the Assignment box if you definitely want to shift all the other assignments forward. Unfortunately, this would mean re-entering the assignment specifics in the new position.
2) Use a decimal number in the sequence number field if you just want to edit an existing assignment. In your example, change the sequence number of assignment #13 to #7.5 to place it in between 7 and 8. The only downside of this method is that if you ask OLLY to auto-assign, it will put 7 and 7.5 on the same day, but if you manually assign, it should be no problem.