I think you have to undo the knots, move them to where you want them and then retie them: at least that's what I did with mine at first. It was a bit fiddly (a bit bit) getting them exactly the same when I retied them. I'm fairly short so I had to do both end to get them short enough.
Then I came across a vid of Nick Woolsey's on colecord in which he suggested another way of tieing it:
http://uk.youtube.com/watch?v=lHXCxOiTyUgMy arms are short enough (total length of my poi need to be 55cm) for me to do the double up method with the length HOP sent me. That works better but beware trying to melt the double cords together - I made a hole in the cord trying it and ended up having to stick it back together with electrical tape. :o