This topic comes up fairly often. Here is a reply I wrote earlier that I have cut and pasted here:
Unhook the old shackle from the centerboard and remove it from the line. Hook the new shackle (on its new line) to the centerboard. Take the cockpit end of the new line, and duct tape it firmly to the shackle end of the old line.
Go up into the cockpit and remove the grommet on the bridge deck that surrounds the hole that the pendent line uses. You need to do this because there isn't sufficient clearance for the bulk of the duct taped joint to pass thru the grommet.
Now pull the new line into the cockpit by hauling in the old line. Once the new line is in the cockpit, undo the taped joint, put the grommet over the end of the new line and reaffix it.
Of course, this whole thing would work in reverse too if you wanted to pull the new line through from the cockpit side down to the centerboard. Either way, no problem. Just don't forget to tie a new stopper knot on your new pendent line!
And I bought my new pendent line with shackle from Rudy at D&R. Not the cheapest solution, but I was having a heck of a time finding a replacement shackle that would fit. Rudy had it and made it easy