Is there a way that I can replace all these prefabs with a different prefab?
I found this code. Replace GameObjects or Prefabs with another Prefab - Unity3D.College
Is this the only way to do it, or does Unity have a built in feature for this?
I’m not clear as to what you are trying to do. If you are changing existing prefabs, the prefab editor will allow you to modify a prefab and save changes back to the original. If you are replacing with completely different prefabs, you’ll need to find the script that references the prefab and change the reference to your new prefab. And, of course rename your prefab to match the existing name, delete the original and drop the new prefab into your castle folder. All solutions are manual labor, no scripting.