I used Sun-In as a teen too, it didn't damage my hair, but it did get my hair lighter, it got lighter and kept getting lighter until I had to dye it back to a dark brown.
ETA: I recommend getting some professional advice and researching online before you do anything, maybe get a consultation with a salon that specialises in colouring, if you get them to breakdown how they would go about the process, that will give you an idea of what you would need to do to achieve your desired results, I know you ain't got no time for any setbacks for 2013