I don’t think there is any hard evidence to back this up one way or the other, but my gut tells me that no, the admissions rate is no different for prospective sophomores than prospective freshman. While there are fewer openings for new sophomores, there are also fewer applicants. Having said that, schools that traditionally take a big chunk of applicants from junior boarding schools may have a lower admit rate for non-JBS applicants.
Of course, you can always ask the admission office if you would be better off applying as a sophomore or as a repeat freshman.