Just undo the cast-off edge and pick up the stitches. Once you find the end and get the last stitch loose, the rest should be easier to pick up again.
If it's wool and has been washed a lot, it may not come loose easily even though it isn't actually felted. Otherwise, it shouldn't be a problem. You might want to use a contrast color and put the same thing on the other end, though, if the yarn has faded at all.