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I've tried this, but didn't work. It was my first tough too. Maybe I'm doing something wrong, I don't know. The wad is in another PC, tomorrow I'll post the code. =/
actor ZombieShot : ZombieMan replaces ShotgunGuy {}
actor ZombieImp : ZombieMan replaces DoomImp {}
actor ZombieDemon : ZombieMan replaces Demon {}
actor ZombieBarrel : ZombieMan replaces ExplosiveBarrel {}
actor ZombieHealth : ZombieMan replaces HealthBonus {}
actor ZombieArmour : ZombieMan replaces ArmorBonus {}
actor ZombieCaco : ZombieMan replaces Cacodemon {+float +nogravity}
actor ZombieSpectre : ZombieMan replaces Spectre {+shadow renderstyle OptFuzzy}
actor ZombieCyber : ZombieMan replaces Cyberdemon {+boss +bossdeath}
Just make sure you define any light frames in the DECORATE itself, not the defs which means you have to repeat everything for each and every replacement.
If Sgt. Shivers wants the replacement actors to actually be of the same type and not just clones of each other (though why that would be needed I'm not sure), I suppose that each actor could be replaced with an actor spawner of some sort and the spawners would always spawn the correct (same) type of replacement.