I untie the knot thing from the bungie, then pull the end cap straight off the rod... There's a bolt attached to it. The trick is to pull the cap, while allowing the string tied around it to slide (loosen) as you pull the cap off.
Still holding the end cap, slide your added weight on (pushing the stopper on the spine down further), and the slide the bolt back in until the cap is flush with the tip of the spine again.
Now, draw the end of the string tight again until the loops around the end cap are tight... Then, just tie the string back onto the bungie however you like, as long as it is tight and won't pop off... The kind of knot doesn't effect how it flies.
If you have any concerns, take a few digital photos before you work with it.