Undoing cast-off: you just need to think about how you did the cast off, and then reverse it. To cast off, you knit a stitch, then pass the previous stitch over the one you just knitted. So, to undo it- grab the stitch you passed over and put it back on your right needle, passing it back over the stitch to get it back in the right position. Then unknit the stitch it was passed over. Then repeat.
PhoenixMoon, all you need to do is carefully pick out your woven tail, then unknit your cast-off. Alternatively, if you think it will look OK, you could just pick up stitches with the new ball of yarn. It will leave a ridge on the 'wrong' side, but if you extend both ends of the scarf you can make it a design element.