I leveled a hunter to 35-ish in late Burning Crusade. When I heard about Lich King, and the DKs being available, I pushed to level 55.
Then I switched to DKs as my main.
In Cata, I eventually achieved Cata-level Loremaster. I've since achieved Loremaster in each expansion (MoP and WoD).
From what I understand of the Legion Transmog system, that means that I'll have access to all of the Plate (and Mail?) quest rewards (and all DK-approved Weapon quest rewards?) for all the quests I have completed. Even the expurgated quests. And the same for the quests I did on my since-deleted Hunter, for Mail (and Leather? and Hunter-approved Weapons?) quest rewards.
Does this mean that we should all send one of each armor type back through all the old quests (including dungeon quests) to unlock all Transmog appearances they have?