The addition of this new position might take you a lot of time though, especially if he is a relatively slow trainer. He will also lag in quality next season. Usually, for slow trainers, you should avoid training for a new position or special ability unless you don't want him to progress beyond a certain level in overall quality (for instance, if you intend to sell the player in the same season, keeping his quality at 99% is better for selling than those of 100% and above). It is also advised not to waste green packs to further train slow trainers as you will use relatively more packs for an equivalent amount of progress for a slow trainer as compared to a fast trainer.