Not really a comparable situation to AC/DC at all. To most people the definitive line-up is Cliff, Phil, Angus, Malcom and either Bon or Brian and four of those are still in the band. They might have had a handful of bassists and drummers before this lineup was cemented but it's irrelevant really. Similar with Iron Maiden. They only have two original members but the 'classic' line-up is clearly the one we have now unless you're one of those oddballs who only likes the first two albums.