학생신분으로 석사 졸업 후 OPT 신분입니다.
최근 I-94를 조회해보니 날짜는 D/S 되어있는데 , I-94 연장이 OPT 기간만큼(1년) 자동으로 되는거죠?
또한 , H1-B로 신분으로 바뀌면 제가 별도서식에 의거 신청해야 하는지요?

네. D/S는 i20를 유지하시는 한 합법적으로 연장이 되는 의미로 알고 있습니다.

Duration of Status (D/S)
On the passport admission stamp or the paper I-94 card, the inspector writes either a date or “D/S” (duration of status). Students and scholars on F or J status should receive a stamp marked “D/S” or “duration of status.” This means that they may remain in the United States as long as they are properly maintaining status and their I-20 (F-1) or DS-2019 (J-1) has not expired. Students and scholars are required to keep the passport entry stamp or I-94 card for the duration of their visit. Make sure to keep it in a safe place so it doesn’t get lost.