You can go to Shattrath and talk to the Spellfire trainer to unlearn Spellfire specialisation (costs about 150g if memory serves), then talk to the Shadoweave trainer to learn that specialisation.
When you unlearn the specialisation, you'll lose the ability to equip the gear that requires said spec (so you won't be able to wear our old Spellfire gear, should you still have it around), but you will be able to make/wear the Frozen Shadoweave set. Also, instead of creating 2 Spellweave cloths per transmute you'll now produce 2 Ebonweave per Ebonweave transmute, which is what you were going for I presume.