I took a picture to better explain what I mean.
http://steamcommunity.com/sharedfiles/filedetails/?id=150811764
You see my new number 2 gets added to the right hand side of the soldier list, the numbers in red are how they will actually appear on the ground combat UI.
If I want him to be number 2 on the ground, then I need to unassign 3,4,5,6 so number 2 will move down to the second spot.
Afterwards I can reassign the other soldiers in the order I want them.