Do you mean would USCGA Admissions decide to switch you after making the initial offer to the Scholar program? If that is the question, then I do not think so. They choose the candidates for the Scholar Program for a reason. Usually they are great candidates who need a little extra help in some area, most often academics. However, I'm not part of Admissions so I can't say that it has never happened.
However, if you are asking about getting an appointment to USCGA the following year, then the answer is yes. ~80% of those who receive a USCGA Scholar appointment will be offered an appointment to the next year's class at USCGA (assuming they reapply). As long as the candidate meets the requirements set out during the year at the Prep school, they are pretty much guaranteed a spot if they want it.