i've only seen it done once so i can't be sure about all of them, but the bearings on the right side of the slave need to be switched, possibly both sides. i believe the top of the pillars need to be switched cuz the slave ones are smaller, the shaft, beater casting, need a bass clamp. maybe a few more things.....
the guy i watched convert them had a single with a broken foot plate and a double with (i think) rusted and seized bearings on the slave, used parts from one to fix the other and get to working singles out of it. this was also 4 years ago so i can't swear to my memory being exactly accurate either....