It's more like an evolution of the base class. Machinist, Dark Knight and Astrologian were newly introduced with the expansion and are just jobs with no initial class. You'll need to reach Ishgard to unlock those which means you'll need, at minimum, one other job at 50 first to complete the main story questline.

Otherwise, classes are leveled to 30, at which point you do a quest to unlock the job aligned with that class. A Thaumaturge becomes a Black Mage. The Black Mage can use all of the Thaumaturge abilities but gains its own set of abilities as well.