I have only done this once, and hesitatingly. First, mount your racket nice and snug on the stringer. Then cut out the crosses and restring them. Probably don't want to make it a habit of doing this.
Only problem I see is that poly dies quickly (2 weeks) if you play often on it. So it there's not much life left in the poly, I wouldn't bother putting in a new cross.
BTW, I typically string my poly mains at 52 and multi crosses at 58.