Alright, gotta swallow some pride and own up to my noobishness here.
In the first few days of WoD I had no deep knowledge of how all of the followers, traits, professions etc worked. Had a general idea, that was it. Early on in doing missions, I acquired the Hearthstone Strategy Guide (+35% xp for accompanying followers.) As, at that time, I thought Fiona's herbalism trait was exclusively beneficial to Herbalists (and myself not being one), I used that Hearthstone Strategy Guide on her, replacing her herbalism trait. Now, I couldn't regret that more.
Question is: can this somehow be reversed (probably not via gm ticket as I have benefited from the party xp buff trait) or is it possible to acquire another follower with the herbalism trait (possibly via the inn?)