Actually, you can learn as many secondary professions as you would like. Each character has the ability to learn *all* of the secondary professions, regardless of having the expansion or not.
As I'm sure you already know, characters may only learn two primary professions. The primary professions are alchemy, blacksmithing, enchanting, engineering, herbalism, jewelcrafting, leatherworking, mining, tailoring, and skinning.
However, you may learn all three secondary professions, if you so choose. The secondary professions are first aid, fishing, and cooking. Simply visit the fishing and cooking trainers (you mentioned already having first aid), and they will teach you the remaining two secondary professions.
The reason you do not have the option to unlearn your secondary profession is because you may know all three at once. Therefore, there's no reason to ever unlearn a secondary profession. Simply pick up all three.
The link below is to the official professions section at WorldofWarcraft.com, and gives more detailed descriptions of each profession, as well as categorizing them into primary and secondary.