Right now the position they learn is semi-random based upon things like their current position, their current ratings, handedness, etc. For example, your second baseman will be waaay more likely to learn third more than he would learn right field.
While not necessarily opposed to adding a choice next year, the worry is that it allows the system to be abused. Like it would allow people to just add all their players to learn catcher until one works for example.
Also, the position someone learns in real life isn't just whatever the team wants (you can't ask all your first basemen to try out SS) because they simply don't have the skillset.
If you want to force a guy to learn a specific position, you can simply force start him there during spring training and he may get the experience.
The difference between the lab program and playing them there in spring training is that the dev program will also increase (if only a little) their underlying defensive abilities, not just their experience at the position. The "spirit" of the dev lab new position program is that the staff evaluates the guy and helps him learn a position that comes naturally to him.
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