Adding the new holes is best done before assemblng the cabinets. It can be done later, but first is easier!
I created a template by marking the spacing of the existing holes, The template is used to drill pairs of extra holes, on both sides one set above and two sets below the upper and lower sets of holes, making a total of twenty-four new holes per cabinet. The shelf pins are used to hold the template in place whilst marking the positions for the holes. A similar template is used to create the new holes for the hinges, halfway between the existing holes, for the twelve shelf layout.
I used a depth gauge to prevent me from drilling through the sides.
The cabinets are also linked together with bolts.