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.
Connemara