The cords are from the boutique, though the last time I was in there, they told me that it was now going to be $18 each time the cord is changed. They also said that they cut off the last cord, so it cannot be reused.
However, this information is contrary to what I'd been told/shown before, so take it as you will.
The cords are tied on, and can presumably be untied. Another SA in a different boutique had previously offered me an assortment of cords to go with the bracelet, should I want them, and said that I could learn to tie them myself or that they would change the cord for me when I would like.
However, I still do not own this bracelet, so perhaps someone else with real-life experience could chime in.