Change attack slots?

Is there a way to change which attack slot the unit uses?

I don't know what you mean exactly... if you mean use first or second attack, I don't think it's possible, but you can make it attack some units with a particular attack (e.g. make attack 2 attack treants only and add "treant" to the type of certain units), also you can give/remove an item with the ability "phoenix fire" to a unit to make it attack with the properties of the item for the time it is in the inventory

I know the answer! yay!

Interestingly enough, the orbs of fire/ice/slow open up slot two for heroes. This is because they give ranged attack, and apparently blizzard decided the only way of doing this would be to open up the second attack slot. So just give them the item.