Not hard to do
assuming you have the Mark II Oday 23 with the centerboard controlled from a line in the cockpit: If the boat is out of the water, this is fairly easy: unfasten the end of the pennant that is attached to the centerboard. On my boat, the end of the pennant has an eye that is fastened to a shackle; the shackle is then fastened to a metal fitting that is glassed into the centerboard. In this arrangement, you just unhook the shackle from the centerboard fitting. In the cockpit, remove the plastic grommet on the bridge deck that surrounds the pennant line as it goes into the deck. You will find that the unfinished fiberglass hole is much larger than you thought. Next, untie the stopper knot that keeps the line from going through the deck. Take the end of your new line that you will attach to the centerboard (the one with the eye) and fasten it to the cockpit end of the old line. You can do this by butting the ends together and using duct tape and/or sewing them. Once they are securely fastened, pull the new line through the hull by pulling on the old line from near the centerboard. The new line will come out of the stub keel; unfasten the old from the new, and attach the new line to the centerboard. Don't cheap out: get a new shackle too.There is no easy way to replace the line if the boat is in the water. You basically have to do the same thing, only underwater.