I have undone a piece just to cast on again. My Blue Herron shawl was almost halfway through the yarn when I noticed that it will never get long enough because it is too wide (if you know what I mean). So I undid the whole thing and cast on 64 stitches instead of 9o something. It should be a better fit this way. I have not reached the end of my unraveled yarn yet (if you know what I mean)..... Looking forward to that milestone.