played the OG version on release. there is a “point of no return” message for the end of the game. If you complete game or save after that point and reopen that save you’re locked into that ending based on what you did prior to crossing the point of no return.
so the playable sections all come before this — so if you missed something after completing you can reload your “Meet [Person] At Embers Bar” save (the point of no return) — go do it with your endgame character and then optionally revisit the final section if you like.
from what I gather on loading up an old save after this patch you’ll have to re spec your character and may have missed minor things, but can start all the new content from your …Embers save.
From a new player perspective you can choose to do the DLC or the vanilla game at any order or at any point- gated by your character level and progression, similar to how Fallout DLC works.