I believe you actually have to complete each mission in both classic and anniversary graphics, which essentially means you have to do the campaign twice. However, since you are at 96% completion, I think you have just one mission left to complete...but the question is, which mission? And in which graphic style?This is not correct. I've never completed any missions on classic graphics as a PC player, and I have it.
That is odd. For the same Halo CE achievement, I was stuck at around 95% until I finally completed a certain mission while in Aniversary graphics (spoiler: it was one of the last 3-4 flood missions, of course). I'm just going by the description of that achievement:
"Complete every mission having seen it in both classic and remastered graphics."
I did initially consider that simply switching to the other graphics at any point could suffice, but it just seemed too
easy to do it that way. Still, if I've made a mistake, then I apologise.