I have begun the extensive surgery required to add length to my Sea and Sand Sweater.
Karen Frisa pointed out--correctly--that I could cut off the corrugated ribbing and then knit up, ending with a two-color kitchener row connecting the old body to the addition. This neatly resolves the half-stitch-off problem of knitting downward off of the body. However, if you have ever cut into two-color knitting, you know that the bottom part is easy to pick up, but the bottom of the stitches on the top? They get all wonky.
To avoid this problem (I hope), I first picked up the stitch and knit into it with blue-violet Tahki Cotton Classic (the right size and nicely slippery without leaving fuzzy bits). Now I will a have nice row of stitches to follow when I do the weaving.
This cotton row also made it easy to locate, cut and remove the plain row between the ribbing and the body.
I'm ready to start knitting--but I gotta tell you, this took hours!