If I have a map item filled out and covering a particular area, will it automatically detect changes in the terrain? Or will I have to revisit the area with a map?
According to official Minecraft wiki:
To record the world on a map, that specific map item must be held in the player's hands while the player moves around the world. The world will be recorded as-is during exploration, meaning that if the world is modified, a player must revisit the area while holding the map in order to update the map's view
So no, the map will not detect changes in the terrain unless you are in the area with the map to update it.