Switching Professions at this point?

I have hit 70, and done a bunch of the "you just hit 70!" content and am a herb/alch. My guild lacks a blacksmith so I was thinking of doing that. My worry is that I have done many of the tutorial quests for profs as alch/herb and have invested all the prof knowledge i have been given into herb/alch and have taken the quest rewards for prof gear of lach/herb stuff. i cannot find any info on what happens if i switch profs, do i lose that knowledge? do i have to aquire new prof gear? will i get a "black smith quest line" to help me start?

TL;DR What do i lose if i switch profs?