There was a huge mess when we did temp workers. Many were getting paid $5/hr less than regular workers without knowing it. Which is fine until somewhere along the way they turn into full hires and that pay rate did not change.
DC lucked out that the women they did it to was a terrible person and got fired for calling out constantly because if she was smart she had a solid lawsuit on her hands.
Anyway, in my experience that pay bump is not guaranteed and I would make absolute sure they tell you what is up.