When I first chose my Alchemy specialization, I misunderstood what I was choosing and only realized it shortly after I had already specialized. But since I already had a big stock of Elixirs in my bank from last time I created a load of them, I wasn't worried.
When it came time to actually execute the switch of the Alchemy Specialization I was very pleased with how easy the whole process was!
How To Change Your Alchemy Specialization
The easy steps are this:
Visit the trainer in Lower City, or trainer you learned your specialization from and choose the "unlearn specialization" option they offer. This costs 150g to complete.
Visit the specialization trainer and pick up the new specialization you wish to have. This is free.
Quest Not Needed For Re-Specialization
The biggest thing I was pleased about was that I didn't have to pick up the Elixir Mastery quest in able to become an Elixirs Master when I had already been a Potions Master.
In looking up information on WoWWiki.com later on, this information was confirmed and they even went so far as to recommend specializing in Potions or Elixirs first, then re-specializing into Transmutation as a cost-saver for becoming an Xmute Master.
Making My First Batch Of Elixirs As An Elixir Master
I brought materials with me so I could put together some Adept's Elixir and some Elixir of Draenic Wisdom for my mage's dungeoning needs, and set to work immediately after re-specializing to see how many of my single-elixir creations would double up to give me more, as a specialist, and was pleased with the results!