But surely they're specialised versions of the current classes? Would be weird to have these two systems running in parallel and be unrelated.

Quests to unlock each job will become available once you have reached a certain rank in the corresponding class.
So you need to be a Conjurer before you can be a Black Mage, I would imagine. It would just feel weird to go back on the current system unless theyre planning on distributing new weapon types to each current class.