It appears the Swallows have been thinking of converting Jamie D'Antona to a left fielder ever since Josh Whitesell joined the team (they both play first base). In fact, it seems D'Antona has been taking fielding practice out in left at Ni-gun in preparation for the conversion (he has also appeared in three Ni-gun games as a left fielder). And while D'Antona joined the Ichi-gun team for evening practice yesterday, he'll likely start the second half at Ni-gun.
As for why D'Antona is making the conversion instead of Whitesell... Fielding and base-running coach Tetsuya Iida told reporters that D'Antona was a better candidate and that the team liked his strong throwing shoulder.