I'm about to return to Dragon Age Inquisition to play Jaws of Hakkon, The Descent, and Trespasser. I completed the main story line, and I would like to change my romance from Josie to Cassandra for these. How can I do this?
Unfortunately, once you've completed the main storyline, romances may no longer be pursued, it seems. From this thread:
All romance discontinues after completing the main story.
The IGN Guide for the final mission (Doom Upon All the World) also confirms this:
You can continue to play after beating the final boss, but some quests will cancel. All quests classified as Inner Circle Side Quests will cancel afterward, including any Sex and Romance quests.
As an aside, if you are playing as a female, you are unable to romance Cassandra at any point during the story.