You will be using that old line to pull the new one in place while the boat sits on the trailer.
It is really petty easy, just lower your centerboards all the down to expose where the knot on the centerboard and pull it out of it's small pocket and cut off the knot.
Wrap some electrical tape around that line just above where the knot was that and burn/melt the end you cut so it doesn't fray and poke a hole through it with a awl so you can fasten a paper clip formed to look like a "S" through that awl hole.
Do the same thing to your new line and tape the paper clip so it won't open on you while you pull it up through the deck fitting and put and old handle on the new line and tie everything off.
Now go sailing....