A couple of nights ago, I pulled out the scissors and chopped off the bottom. Then I set about picking up stitches and picking out little bits of cut-up stitches. It's not as hard or as scary as it seems; in my experience, the surgical patient has never unraveled uncontrollably leaving me with a tangled-up mess. Note that I knit only in natural fibers, usually with wool content. If you knit with a slippery man-made yarn, all bets are off when it comes to this kind of modification.
After I picked up the stitches, I just started knitting downward. I added almost three inches and then finished it off with an inch of garter stitch, like the original design. Easy peasy.