Hi, I recently started a gameplay where I created townies for a empty world. For storyline purposes they all had specific careers instead of just having nraas assign them but I noticed later on that their careers had changed. Is there anyway that I can stop nraas from changing their careers?

Are you using NRaas StoryProgression? If so, then SP may have reassigned the sims to careers that better suited their traits and lifetime wishes. To stop that, click on City Hall and select:

NRaas > SP > Town Options > Career: Allow Find Job > False

The game can still add unemployed sims to certain careers as coworkers, but anyone already in a job should stay there. This shouldn't affect your ability to assign jobs with MasterController, just what SP can do on its own.

In the same location, you can set Career: Allow Progression to False if you want the sims to stay at the same level.

If you only want certain sims to stay in their jobs rather than the entire town, create a caste, add the above setting(s) to the Caste Options, and add the sims you want to that caste.

For some reason the not so great Showtime professions can replace NPC careers and not other professions from other packs.