Although there is probably already stuff regarding changing sides of units, it is easier to have another thread based on this subject rather than dig through mountains of topics. This is just my way, and you could probably figure it out yourself if you wanted to put forth the effort or just messing around with the grouping tool. With this technique and correct use of group ungroup commands, you can have two East guys kill eachother, civies fight the west, whatever:
-Create a unit with rank colonel on the side you want your "real" unit to be on. Create this unit in the water and set his health to zero, or not. Either way just make him in the water so the player (or players) don't stumble upon the body.
-Create your "real" unit whereever you wanted him, set his properties to "none" (or he'll spawn in the water), and make sure he is NOT the squad leader (why you made your water unit a colonel), and group him with the water unit. Now the AI will act as if this unit is the side you set your "water unit" to. Remember that rule: the side of the group leader is the side of the entire group. Now, you can even get the West teams and East teams to work together!
If you use waypoints such as "join and lead" or use the ungroup scripts, you can easily make units switch sides in the field!