I have been looking at pics of the various Antminer boards. They change quite a bit. The question that remains unanswered is, in my mind, whether you can make minor modifications to an S3 or S4 board to upgrade it. It is not a matter of recreating an S5 in the process, but rather increasing the performance capabilities, reducing the power consumption, while also maintaining a cheap re-manufacturing process, enough to compete against the far more costly newer hardware versions that emerge every year.
One would have to look at other hardware vendor boards to see if they are duplicating board layouts in a manner that lend themselves to minor upgrading processes. ASIC chips are cheap to come by, after the hardware vendors release product. Modifications of past board layouts may be as simple as replacing diodes, resistors, and capacitors; I do not know.
With the advent of the new PCB printer, this could be a viable future business model to pursue. I am just thinking outside of the box at the moment, but if you could take new emerging technologies, replicate them to fit older housings, you could be in the upgrade business for some time to come, keeping miners up-to-date in a manner that is actually affordable, while also generating a healthy profit monthly. I am not so sure you really need to be an electrical engineer to pull it off either.