The Forgotten Realms wiki page on succubi tell us (specifically in footnote 1) that in 1e, 2e and 3.Xe1, succubi were chaotic evil demons, but then were retconned to be lawful evil2 devils in 4e and have now just been made into generic neutral evil "fiends" in 5e, presumably in an attempt to avoid contradicting any previous editions' lore.
Unlike with the Shadar-kai, I believe there was supposed to be some kind of canonical in-universe lore reason as to why these demons became devils. What was that reason? I assume it appears in some 4e material somewhere? I'm only really familiar with 5e material...
This question used to have another part to it, but that been split out into its own question; see: What is the in-universe explanation for why succubi, who were demons, became "neutral evil fiends" in 5e?
1 Actually, the footnote on the Forgotten Realms wiki page only says 3e, but I know it was still true in 3.5e because of Neverwinter Nights 2, which was a video game based on 3.5e. In this game they were considered demons, which is incidentally my introduction to D&D and why I consider succubi being demons to be what they "should" be.
2 I say "lawful evil", because that's what a devil's alignment is, but I'm aware that 4e changed the alignment system, so it might not be so accurate to claim they were "lawful evil" in 4e, but at the very least, in the context of D&D overall, they would have been considered lawful evil all the time they were considered to be devils.