I'm sure you've looked closely, but doesn't the slot in which the sail slides ..er ... slide have a wider bit at the bottom?
If not, how do you get the sail off?
'Cause what you really have to do, I bet, is take the sail off using that wide bit, attach the new slide in the appropriate place on the sail luff, and put the thing back on again.
I speak without having any idea how your mast is configured, of course. You may have some other way to get the sails
on and off. In which case, use that.