Not possible to unlearn Weaponsmithing to learn Armorsmith

[Fawcan]

If I speak to the guy who teached me Weaponsmithing and choose to unlearn it he does nothing.

[Mopar]

You cannot unlearn specialisations in Vanilla. If you want to change to Armorsmith, you have to unlearn the entire Blacksmithing profession, pick it up again from the beginning trainer, and re-level it.

[Georgerr]

You cannot unlearn specialisations in Vanilla. If you want to change to Armorsmith, you have to unlearn the entire Blacksmithing profession, pick it up again from the beginning trainer, and re-level it.

wow, srsly ?! O.o

[Mimma]

Seriusly, yes.

What you want was introduced in Burning Crusade:

Since the release of The Burning Crusade, you are now able to learn any of the weapon subspecializations, by talking to one of the appropriate three masters in Everlook, and asking them to teach you (though you must still have completed the weaponsmithing quest, as above). The quests, however, still rewards the same plans. These abilities can be unlearned for 100Gold and another weapon subspecialization can be learned in their place.

Source: http://wowwiki.wikia.com/Blacksmithing_specialization

[AverageJoe]

Vanilla

Not for the faint of heart.