I have to agree with PoM. There is a problem.
One of the principle rules about Civ4 combat is that, even in Stacks of Doom, even with collateral damage, it is still a one-to-one fight. It makes sense for a withdrawing unit to hide behind the lines of their allied infantry, but to have another defender take its place is too much. After all, doesn't the attacker have the choice not to pursue? By replacing the defender like that, you're essentially allowing a defender to fight 3:1 against the attacker (even though you limited it to 3 in version 2.0, three to one is still an unfair ratio), while the attacker never has this option. It disbalances the one-to-one nature of Civilization combat.
I wanted to use this mod for... what's the term?... "Vanilla with sprinkles" playing. I don't want drastic changes to the rules, but this feature seemed fair and clever. Except that replacement part.
What's wrong with letting the attacker choose the next unit to send in? Maybe, in the case of a withdrawal, the "One combat per turn" rule doesn't trigger, so the attacker will have the option to pursue anyway?