Minecraft wiki entry for Updating_old_terrain, associated text: Open your world in MCA Selector
Click on "Tools" -> "Filter chunks"
Replace the line xPos <= 100 AND xPos >= -100 AND zPos <= 100 AND zPos >= -100 with InhabitedTime = "0", press Enter then click "OK"
Now the generated chunks that no player has ever been near are selected. You can delete them by hover at "Selection" -> Delete selected chunks -> OK