Rashi comments on "Vayavo Moshe vayisaper la'am es kol divrei Hashem..." (24:3) that Moshe told Bnei Yisrael the seven mitzvos Bnei Noach and the mitzvos they had been given at Marah. Ramban disagrees, as the word "Va'yisaper" usually means relating new information. Bnei Yisrael already knew the seven mitzvos Bnei Noach -- why did Moshe need to teach it to them again?
R' Yosef Engel (Beis haOtzar, Avos, towards the end of part 6) raises the question of whether the chiyuv on Bnei Yisrael to observe the seven mitzvos Bnei Noach remained the same post-mattan Torah as pre-mattan Torah, i.e. mattan Torah simply added new mitzvos, or whether mattan Torah transformed everything, i.e. it was a package deal with a completely new set of chiyuvim.
The Rambam Hil. Melachim ch 9 writes that Adam haRishon was given the first six mitzvos, Noach was given a seventh, and then:
וכן היה הדבר בכל העולם, עד אברהם נצטווה יתר על אלו במילה, והוא התפלל שחרית. ויצחק הפריש מעשר, והוסיף תפילה אחרת לפנות היום. ויעקוב הוסיף גיד הנשה, והתפלל ערבית. ובמצריים נצטווה עמרם במצוות יתרות, עד שבא משה רבנו ונשלמה תורה על ידו.
The Rambam uses the historical precedent set by Adam, Noach, the Avos as the basis for the chiyuv to observe certain mitzvos, to which Moshe then added and completed the Torah. The Torah did not change chiyuvim that already existed, but simply added to them.
I would like to suggest that Rashi disagrees. The reason Moshe related to Bnei Yisrael the seven mitzvos and those commandments they had already been told at Marah is because these were not the same commandments. Mattan Torah fundamentally transformed thosee obligation which existed from simply being ben Noach laws to being a mitzvos -- a qualitatively different legal animal.
(It was noted that I should have mentioned the Rambam in Peirush haMishnayos in Chulin that says the chiyuv of gid hanasheh is because such a mitzvah was given at Sinai, not because such a a mitzvah was practiced by Ya'akov. You need to work out how to fit that in with the Rambam in Hil Melachim. I also want to mention that it's worth seeing a sicha of the L. Rebbe in Likutei Sichos vol. 33 p.26-32, esp. the last section where he explains that even if the Avos did not have a din ben Noach, full fledged geirus did not occur until mattan Torah because only post-Sinai was there this new qualitatively different level of chiyuv (and hence qualitatively different relationship with the RBS"O) created by mattan Torah. Maybe more on this after Shabbos.)