Swapping units during battle is an interesting problem, both in game play and apparently in real life.
https://youtu.be/croWDsDhgPo I am sure we all enjoyed the way HBO portrayed the swapping of lines within a maniple, but that is not the same problem.
https://youtu.be/J7MYlRzLqD0
How do you swap your units during a fight? Do you first send in the relieving unit, then when it is engaged pull back the tired unit? That sometimes works, but not always, even in vanilla. It strikes me that it should be risky business withdrawing from a fight.