It can sometime take a very long time, depending on how comfortable they are in that position to start with, how versatile they are, and how much they play in the position you want to train them in. For example, I trained Phil Jones to be a natural right back at Man Utd, I wanted cover for Rafael, but due to injuries and Rafael having bad form, Jones played about 40 games at right back in my first season, and it was a few games into season 2 that he finally became natural at right back. Bearing in mind Jones is already accomplished at right back, it took over a season, with him playing 40 games, and constantly being on the right back retraining program. So be patient, it will take a while.