I am knitting a baby's sweater by sections - I have just finished the back piece, and the pattern says to slip all those stitches onto a stitch holder. For the next section (left front) it says cast on 27 stitches - but my working yarn is still attached to those stitches on the stitch holder. Do I cut the yarn on the stitch holders in order to cast on new stitches to my needles? or just keep working onto my needles?