I'm unsure if this has been mentioned yet, but I am currently playing on Nighthaven as a Horde Feral Druid. Went to TB to train a new rank of Faerie Fire (Feral)(Rank 3). When I did, it untrained Rank 2, I have the option to re-train it at the trainer, and it removed Rank 2 from my spellbook. I tried training Nature's Grasp from the Balance tree, to see if it would derank/unlearn the rank 2 versions, and it did not.
Here's showing where I trained FF(F)(Rank 3):
Showing where Rank 2 is no longer in spell book:
