Not necessarily. You could also change <DefaultUnitAI> and the allowed UnitAIs in <UnitAIs> of the Units. (e.g. UNITAI_DEFENSIVE) But you should really know what you are doing. Otherwise you will probably totally mess up / cripple AI. Going into the DLL will of course give you a lot more possibilities if you know what you are doing. AI behaviour is not really a topic a modding beginner would usually touch.