In my opinion, this is part of what a post-bac is for. You should absolutely feel free to change your mind. Much better to do it during this stage than later on in the game! I would say that about 25% or so of our post-bac students come into our program undecided or switch goals mid-way through the program and we encourage that kind of exploration. Work with your program's pre-health advisor. It is great to see a student finally "click" with their calling and get motivated!
In your essay, you can say that part of the reason you want to do a post-bac is to build certainty as to your goals and that a deeper exploration of these two x and x career choices is part of that. I don't think you need to shy away from mentioning your ideas; just be sure to come accross as excited to learn about the options rather than wishy-washy blah.
