No, thankfully. It's very appropriate (and strategic) to use the same essay with modifications. Ensure you explicitly address why a prelim year will advance the rest of your career and how you will contribute to the training program as a future specialist.
When you use a very similar essay, you can be honest about what your professional goals are. After all, the reader knows you're applying for a one-year position anyway.
Remember that many preliminary/transitional year programs are eager to match residents who are moving onto competitive fields. In general, those applicants will have strong USMLE scores, evaluations, and clinical skills.
