There is a gemara that says ???? ??? ????? ?? ???. Only four people in the history of world died without doing any aveiros whatsoever. The only reason they died was because of the g’zeira from Adom and Chava. One of them is Amram. (The others are: Binyomin, Yishai and Klav ben Dovid). How could Amram have married his own aunt? If you will say that it was before matan Torah, so then the whole gemara looses its punch.
A friend of mine quoted a Nefesh HaChaim (Rav Chaim Volozhin) who explains the fact the Avos kept the Torah was not the same way that we have the Torah. Yes, the Yidden kept Shabbos and mitzvos. They knew the Torah, but were allowed to make their own calculations. When Yaakov saw that he was given Leah, he married her sister as well because he knew that the world would need a Moshiach ben Yosef. Maybe you can say, that Amram knew that from him together with Yocheved the “goel” was to be born. So he was allowed to marry his aunt. As soon as Paroah made a decree that all boys must be thrown into the river, he knew that even if a “goel” was to be born from him, he would have to kill him, and hence had no reason to be married to his aunt. So he divorced her. Miriam came along and said that its your job to bring the “goel” onto earth, from there on Hashem will worry about it. Ofcourse, once we were formally given the Torah on Har Sinai, we can no longer make our own calculations.
To take this one step further. Eventhough, Yocheved remarried, (and even if Amram had a din kohen), he was allowed to be machzir grushoso, because we was allowed to make such a calculation.
(Please note: this is my own p’shat based on what I heard in the name of the Nefesh HaChaim, but for people who are more learned than me, it may be a long bubba mayss, but from what I was quoted, kein nire’e li)