Moses received the complete Torah during his third ascent to Mount Sinai. What Torah did the Levites study during the building of the pyramids at the time of the exile in Egypt. The book of exodus had not taken place yet, so was it a partial Torah? Also, what Torah did Abraham, Issac, and Jacob Study? Was it again a partial Torah, which only became complete after Moses returned from studying the Torah on Mount Sinai with Hashem?
The traditional way of explaining the Forefathers keeping the entire Torah is that since the Torah is the blueprint for the world, and they saw the world, they could take the world and construct blueprints. This means that they could figure out what the Mitzvot are. Their descendants, including the Levi’im, now had a tradition from them what and how to observe the basic Mitzvot, even though they were not yet commanded about them. The Forefathers were commanded concerning certain Mitzvot, such as Brit Milah, and gradually observed others as commandments. Moshe at Sinai was commanded about all the Mitzvot, although many were not applicable until the Jews entered Israel.