if you change the players training schedule, his stats will drop first before improving (if you change him from Target Man to a Poacher schedule)
they will drop if you ask him to learn a new move as well as a new position, you will see in the training tab that 20% of his workload is focused on him learning the new move and position, meaning he will only be on 80% of the training, hence is stats will drop.
for all young players under the age of 21, DO NOT teach them moves or change their position as this eats away at his PA points, rather let the PA points get spent on his development (stats wise) during this time, tutoring can be used to teach him PPM's...