How can I trace a pattern from a top with darts?
If the dress is ready for replacement, you can remove the seams. You don't have to remove all of them, just the ones so the fabric can be layed down flat. I wouldn't recommend removing seams if you still want to wear the dress.

Another option is measuring: draw the pieces of your dress on paper as good as you can. Cut them out. Then put on your dress and see if the pieces you've cut out of paper, cover your dress. If you've done well, you'll also have the dart on the right place. When the doesn't fit correctly yet, cut off what's too large and add paper where your pattern is too smal. You can just attach a strip of paper with some tape, no need to redo the whole piece.

If all else fails, you could try to find a pre-made pattern that resembles your dress, and maybe adjust some parts (e.g. length, neckline).