Border Guards you play reveal 3 cards and discard 2. (It takes all 3 being Actions to take the Horn.)
How I understand it:
There are two ways to interpret this. Either this is actually a modification of Border Guard's played effect (i.e. "When you would reveal cards with Border Guard, reveal 3 of them. When you would discard cards with Border Guard, discard two of them.") or it is a replacement of Border Guard's effect (i.e. "When you would play Border Guard, instead of following its effect, reveal 3 cards and discard 2.").
If Lantern were a replacement, then it would, in fact, be able to replace Enchantress (or at least you would get to choose, I think).
However, we know for a fact that Lantern is not a replacement. If it were, Border Guards would cease to give +1 Action.
Therefore, it must be a modification, and therefore it does not replace Enchantress.
Way of the Chameleon, on the other hand, is a replacement followed by a modification. The fact that it is a Way makes it a replacement. But, it tells you to "follow this card's instructions," so most of the time the replacement section has no effect, except with Enchantress. The replacement is then followed by a modification, but Enchantress by this point has already been replaced.
The real question: How does Lantern interact with Way of the Mole. Way of the Mole tells you to discard your hand. Does Lantern's "Border Guards you play discard 2 cards" mean that a Moled Border Guard can discard only two cards instead of your whole hand? I imagine you can choose whether Lantern's modification comes before or after a Way?