Headmaster Achievement - replay mission

Can anyone confirm if I can make a run for this, by just replaying reach mission on my 100% save, or must it be a fresh save? The details just say to do each mission…

Thanks

https://forums.halowaypoint.com/t/headmaster-achievement-not-unlocking-after-completion/537656/34

Thanks. I read that as well, but wasn’t sure if it applied to someone who’s never tried it before. Unless the entire save file pre-update cant be used at all.