when it says a player is accomplished at a position, how much of their ability is lost compared to when they play in their natural position?

no one?

I'm not sure it's quantifiable, neilb242. Accomplished is a good rating, so you should not incur too great a penalty, if any.

Indeed, there's not even a great amount lost if the position is ineffectual (just make sure the player has the attributes to play in that position, or you will be proved otherwise).

I guess the only real thing you have to note is that the player may lose knowledge of an accomplished position, but not a natural one.