This is totally dependent upon how the HS designs the class. There are many permutations in the way Calc BC is taught. If Calc BC is taught with roughly half the time devoted to AB topics and half to BC topics, then I agree, nothing special needs to be done to prepare. However, for schools that offer Calc AB as a year-long course and Calc BC as the next course in the math sequence, jumping from pre-calc to Calc BC without preparation is a recipe for disaster.