When diii comes out, i'll let you know. But, I'm gonna assume you mean d2 enchant, so, if a person tells you not to enchant him because it lowered his dmg, stop enchanting him, and leave him to his ignorance. I always gladly accept enchants, no matter the lvl
Well if they were enched by a lower slevel, but significently better synergised, ench, I guess the highest level ench would override despite being lower damage. Not certain.
He probably believes that fire buffs can't stack, so your +8-10 fire enchant (probably not your actual amount, but I'll use it to prove a point) will push out his +300-400 fire damage.
I'm quite sure a higher level enchant can't be overwritten; you won't see anything happen at all if you cast the weaker enchant. I would know since I screw up prebuffing so much.
You don't even get the animation if you, say, ench with a skill shrine or prebuff then try and re-ench without. The only question is whether it bases it on level or damage, and seeing as ench has a major ar component I'm thinking the former.