some time ago when i was an aktiv minecraft player i used to have a verry nice mod that alowed me to change the mob spawned by a monster spawner.
You just rightklicked it, then you had a menue to choose the mob you want.
Unfortunatly i can´t find this mod anymore.
Maybe some of you can crate that mod/update the old mod or send me a link to the updated version.
It's possible to manipulate Mobspawners even without forge. You just need to take the egg of the monster you want to spawn and then right-click the spawner (in creative mode). I hope this is useful.