I'm also an out-of-field applicant, and the prereq courses that I took generally did a good job of introducing the topics and vocabulary for those who had no prior experience. However, my undergrad was in Linguistics, which has a LOT of overlap, so I was already on a firm footing with language development, cognition, grammatical structure, etc., so it would probably be a little tougher for someone with no prior experience. I think what's most important going in is to have at least a basic grasp of how language works in a typically-developing person. Maybe explore a bit on linguistic structures in the brain (Broca's area, Wernicke's area, etc), a bit on different types of speech sounds and how we create them, some basic anatomy of the vocal folds, and how young children develop language. I don't think this would take too much effort, just enough for an overall grasp of Language as a whole. I took a couple undergrad courses in communication disorders, which may or may not be a part of a post-baccalaureate program but are often required before beginning a grad program. My current post-bacc program does not go into great detail on various types of communication disorders, so learning more about them will be very helpful for grad school but probably not super important for prereqs.
I'm a career changer as well and just recently accepted into grad school!
I don't think it's necessary to study before the pre-req classes, most post-bacc programs will start out with the fundamental courses and then you will advance to more in depth classes, so they will adequately prepare you. If anything I would focus outside studying on the GRE since that's personally one of the more challenging parts of an application (at least in my opinion). Do you know where you're planning on taking the classes? I did the online program with University of Wisconsin Eau Claire and really loved it. I know there are a few out there but that one worked best for me (since it was online) and seemed to be more affordable than some of the other ones. Good luck!
