I have two machines, one with a 10mm plunger, and a 15mm plunger. Increasing the shot size depends on the size of plunger you have. They came in three sizes, 10 (1.5cc), 12(2cc), and the largest at 15mm(2.5cc).

If you pull the barrel, the plunger slides right out. The plunger fits into a matching, removable sleeve (IIRC, roughly 75-100mm long), so they are the only two parts which require changing. You might even be able to fit in a set larger than 15mm, but your injection pressure will be reduced, and you will still have to mind the surface area so you don't blow through the parting line.