Is there any way that you can expose the bottom of the keel on your boat to let the board down enough to get at the pendant line shackle? That will be a harder task than actually replacing the line. The line can be replaced by tying a strong string to the end of the pendant line from the cockpit and pulling it out through the bottom of the keel.
There is a small single sheave way up inside your centerboard slot where the pendant line rides through and you need to make sure that the line goes through it if you plan on doing it a different way than I just described. To replace the line, you just tie the end of the pendant line to the string from the bottom of the keel and just pull it up into the cockpit. Then you can tie a figure eight knot in it and connect your shackle to the centerboard. It's an easy job if you can get at the bottom of the keel.
Joe