Hello,
I changed mines last year (the short lines) . It is easy to do, once we understood the system.
There are 3 sliders in the boom: one on the port side of the boom for the second reef; two on starboard side, the most on the back for the main sail back point and in front of it the slider for the first reef. Line lengths need to be correct to avoid conflict between these two sliders.
Access is done by the back of the boom after removing the 3 screws. Sew the new lines to the old ones before pulling back the sliders.
For the length, it is about the length between the boom and the reef point once the sail is fully up, plus extra for the knots, when the sail is up the reef sliders are close to the mast.
The slider with the cable is for the back of the main sail.
The reef line is blocked with a knot in the slider (difficult to make, space is short).