I have heard of players changing position.
It appears that the position is calculated based on relative strengths of skills. For example, PG appear to have better passing/handling/driving skills than shooting skills. If you search the transfer market for a PG with say mediocre passing, he will have very poor shooting and inside skills. But if you search for a PG with respectable passing, he might have average JS, or poor shooting. If the shooting is just a bit worse, you could probably convert a PG to a SG by training JS and JR.
If you search the transfer market for a SG with respectable passing, he will have very good shooting, because it has to be stronger than his passing for him to be a SG. You could probably convert him to a PG by training passing.