Reserves are different. In the reserves, when you join, you are joining for a specific job. As far as changing your rating, that would be up to your chain of command and the reserve rating force manager. On the active duty side, you are allowed to take the ASVAB again in order to qualify for more jobs, but I can't say for certain if you can in the reserves. That would be a great question for your recruiter, and they can find out for you.
PO1 Kevin Couture