I think they would almost certainly be using CAD in the design stages and probably 3D printing for prototypes. The quick turnaround times and relatively low cost would make these pretty attractive. CAD would be useful for optimising your parts sprues too.

Wikipedia says injection moulding dies are produced through CNC machining or electrical discharge machining. While not toy specific, I don't see why toy moulds wouldn't be made in the same manner - probably faster, cheaper and more consistent than other methods.

Replacing a mould designed this way would probably be fairly easy to do, though the cost of replacing them in the quantities required for large scale runs would still be quite high.