According to Bitmain's ads for the S2 kits back when they had them, they stated that both S2 and S2 kits would be upgradable having the ability to swap out the boards with new boards containing the new chips.  Based on the fact that Bitmain said the upgrade would bump the S2 up to around 2Th/s and considering the new S3 boards push about 240gh/s(so 10 boards would be around 2.4 th/s), I would put my money on the fact that the upgrade to the S2 will be a hardware upgrade.  Also considering the new chips run at around 0.5w per gh/s a stock upgrade probably won't require a PSU grade(although if you want to OC you will probably need to).  Anyways those are my thoughts.