Make a dummy spell that doesn't do anything, but you cast it on another unit. Then create this trigger:
Events
--Unit - Hero Begins casting an ability
Conditions
--(Ability being cast) Equal to Ability
Actions
--Unit - Replace (Target unit of ability being cast) with a Weak Unit using The old unit's relative life and mana
--Set Target_Unit = (Last replaced unit)
--Countdown Timer - Start Timer as a One-shot timer that will expire in 1.00 seconds
Then make this trigger:
Events
--Time - Timer expires
Conditions
Actions
--Unit - Replace Target_Unit with a Footman using The old unit's relative life and mana
Hero = The unit with the ability. Ability = The dummy ability. Weak Unit = The unit you want to replace the enemy unit. Target_Unit = Unit Variable. Timer = A timer to count down the length the unit is replaced. You can change this to whatever length you want it to be for.