the retraining depends on the basic ability of the player in that position before i think.
like post #4 said, if your player has no ability in playing that certain position, retraining him into that position can make him unhappy and the training may not be effective. you will also need to consider the determination of the player. player with high determination will have bigger chance to be trained into another position.
i suggest that you should only retraining player if he already has one level below acomplished in the new position.. i dunno about pato though, but i think he is a good striker, and milan has kaka for amc position..