This might not be relevant, but eons ago, I went from a first year HS-level class to auditing a first-year, second-term college-level class in a foreign language without too much academic difficulty. (Most of my challenges were in executive functions, at the time, as I was also a preteen without exceptional EF.) If AP is supposed to be the equivalent of entry-level college, then one would imagine that all relevant grammar and most vocabulary would still be reviewed, but more quickly, under the assumption that it isn't the first exposure.
