yekke2:
Many people are under the impression that the firstborns will revert to their original status of Priesthood when the Third Temple is rebuilt. The Brisker Rav zt”l held that this was blasphemous [and contravened the principle of faith that the Torah will never be changed nor will there be another Torah forevermore] and incorrect.
Any idea where that Brisker Rav is brought down? Because as you go on to note, the Ohr HaChaim seemed to have accesses to a Medrash that has since been lost that says the Bechorim will join the Leviim when Moshiach comes. (Although not, it seems, the Kohanim in the actual avodah.)
(Full disclosure – I am a bechor, so I have an obvious negius here 🙂