I dropped Engineering mid cata for blacksmithing, because at the time I was tanking and the tinkers were unreliable, so I picked up Blacksmithing because it gave the most benefit and allowed for stat customization.
Anyway, I'm planning on picking Eng back again for MoP, but I'm wondering if all the recipes I trained while I was an engineer are lost or if I keep them. This may seem a stupid question at first, but there are seems to be some "memory" of being an engineer, like still having the mount and BoP schematics still dropping for me. If I don't have to get all the recipes back (Jeeves, for instance) I plan on picking mining before leveling in MoP and then leveling Eng at 90, but if not I might as well start it now.