I leveled up a new character for the expansion and would like to work on unlocking the Zandalari Trolls now. Is it too late to go back and pickup those old questlines to get the necessary achievements, or do I have to start a new character and do it before starting the expansion?
I can’t seem to get a clear answer on that and am struggling to find access to the quests all the guides mention.
Any help is appreciated!
You can do it any time on any character who’s a member of the appropriate faction.
Edit: I understand your confusion, though, because BfA has kind of an obscure lead-in. I haven’t done it as Horde in a long time, now, so my memory of the details has become hazy.
You have to do the intro quest from (I assume) Orgrimmar before you can start doing the quest chains in Zandalar, but I don’t recall exactly where that starts.
Ok thanks! I just wanted to make sure that by doing the Dragonflight storyline it didn’t lock me out from starting that old questline or something like that before I waste too much more time trying to figure out how to get it started 
Nope! You’re in the clear. If I knew more about how the horde started BfA I’d let you know, but honestly I had a hard time finding the start for Alliance on my Evoker recently…and I’ve done alliance-side BfA stuff on about 10 characters.
It’s just weirdly hidden in general lol
I think both factions actually start BFA by having Magni ask them to visit him in Silithus. That leads to a questline about the Heart of Azeroth and explaining what Azerite is, and then from there you get told to go back to your faction capital and then to the new zones.
(I have a quest like that chilling in my quest log as an evoker; if evokers can do it, I’m pretty sure anyone can.)