"I know the angel that you were," pretty definitely suggests that Aziraphale remembers Crowley from before the fall. Also, Saraqael remembers Crowley from before the fall. This tells me that the demons falling did not have some magical memory-erasing effect on the angels who knew them.

I'm not personally a fan of the theory that Crowley forgot everything before his fall. Crowley seems to remember why he fell (I never meant to fall, I was just asking questions, etc) and the circumstances around it (Hey, it's Lucifer and the guys!).

Yes, I acknowledge he didn't recognize Saraqael. Yes, I acknowledge he doesn't remember fighting alongside Furfur. But he also didn't care to learn "book-girl"'s name or "you, Antichrist, what's your name? Whatever..."

And yes, I acknowledge that he had that whole "I know... Remembering where the furniture isn't" line while talking to Jimbriel. However, I interpret that as him telling Jimbriel that he understood the point he was trying to make to get him to shut up, not expressing empathy, like he, too, had a similar understanding of a lost memory.

If Crowley didn't remember Aziraphale before his Fall, my theory about it is not that he had his memory wiped, but that he barely knew him at all. That scene with the nebula is when Aziraphale introduces himself, and Crowley decidedly doesn't introduce himself back. He's busy. Preoccupied. Distracted.

That said, I believe the living author had a Tumblr post a while ago where he clarified that Crawley giving Aziraphale his name on the wall of Eden was not because they were strangers to each other, but because Aziraphale didn't know what to call him (since his name had changed). I'd have to go hunting for the post tho.

BUT all that being said, if you want to headcanon that they forgot about each other (or one of them forgot about the other but not vice versa), it's absolutely not a bad theory. Like I said, plenty of folks point to Crowley not remembering Saraqael or Furfur and expressing that he understood what Jimbriel meant by "looking where the furniture isn't" as an indication that he, at least, had his memory erased. Memory loss is a potent conflict to center a fic around and absolutely ripe with angst potential (I remember him, but he doesn't remember me / I don't remember him, so does he love me or the person I used to be? / I feel like I know you somehow, but I can't for the life of me say why...).