If a couple was married in Mitzrayim, but their relationship was considered - after Matan Torah - to be ervah, could they remain married? For example, Amram (father of Moshe, Aharon & Miriam) was married to his aunt, Yocheved bas Levi. After Matan Torah, their relationship was asur. Were they allowed to remain married (being grandfathered in) or did they divorce?
The Daas Zekanim Mibaalei Hatosfos, Bamidbar 11:27, discusses Eldad and Meidad: Eldad and Meidad were brothers of Moshe, for when the Torah was given and the arayos (plural of ervah, it means incestouous relationships) all separated [from their spouses] at it says 'vayishma Moshe es ha'am bocheh lemishpechosav' (and Moshe heard the nation crying to their families) - the drasha is "about their families" [(i.e., wives) becoming forbidden]. Also Amram the father of Moshe separated from Yocheved who had been his wife, and he married another women and bore from her Eldad and Meidad.
He holds they were not allowed to remain married. (He can hold that ervah depended on the kedushas haprat.)
The Meshech Chochma (Devarim 5:27) states: The Chasam Sofer (Avodah Zarah 63b) is unsure as to the source that a ger who converts is like newborn child. To me it is simple that undoubtedly among those men who left Mitzrayim many women (wives) who were from those [relationships] that are not forbidden to Bnei Noach. And Amram is a case in point, he was a gadol hador married to his aunt...these men who were married to their relatives would have had to separate from their wives [but this pasuk says "return to your tents", i.e., your wives, implying that they remained married to their wives] and thus we see that a ger who converts (meaning the Jews at Matan Torah) is like newborn child.
He holds that they were allowed to remain married because the geirus at Matan Torah made them be considered not related.
The Gur Aryeh, Breishis 46:10, says that "a ger who converts is like newborn child," did not apply to the Jews at Matan Torah since G-d lifted the mountain over them as a tub [and pressured them to accept the Torah], so they could not be considered like a newborn. But regular geirim are considered as newborns.
The Gur Aryeh uses this idea (the same idea as the Meshech Chochma) to explain how Yaakov Avinu married two sisters - after geirus, they were not halachikly sisters. The famous Ramban (Breishis 26:5) says Yaakov married two sisters because he only kept the Torah as "mi she'eino metzuve v'oseh" (not out of an obligation) when he was in Eretz Yisrael, and he married Rachel and Leah outside of Eretz Yisrael. (This Ramban needs some discussion. His question includes how did Amram marry to his aunt and how did Moshe build 12 matzevos. But in the midbar, after Matan Torah, they were obligated in the Torah, so having not yet entered Eretz Yisrael would not have allowed them to not fulfill a mitzvah. Perhaps Moshe's building the matzeivos was a hora'as shaah. Still, it is unclear if the Ramban would agree with the Daas Zekanim or the Meshech Chochma about Amram remaining married to Yocheved after Matan Torah.)
(I would like to acknowledge that I first heard this Gur Aryeh, the above Meshech Chochma and the Ritva Kesuvos 11 [in the first post about Avos as Jews] from R. Aryeh Shtern at KBY in Sivan 5764.)