Theoretically, if you are just using one circuit, you won't have to change the wiring. be wary what else the circuit might be used for though. Don't end up trying to run your lights or hairdryer off of 240 (single phase is actually 120V, not 110V, interesting fact). You will be using two phases so you should replace two single breakers with a double. You should use a different outlet to prevent accidentally plugging something in that you didn't mean to.
Your PSUs may be dual voltage meaning you just plug them in. Some have a switch to switch between the two. Some are only capable of one or the other and you would either have to use an adapter or replace them (replace may actually be cheaper). It should say on the label on the PSU (Though I once had to replace a 120/240 PSU that would not work on 120).
Whether it makes sense is something you would have to work out. It would depend on the circumstances you are working in. How far from the breaker boxes are your rigs? Are you in a home (presumably) or a business? Do you care that you might need to switch it out when/if you stop mining or move? Could you just use an extension cable instead?
The current 120v/20a circuits were purpose-installed for mining. I just didn't realize that the machines would be pulling so much amperage vs. wattage (about 1200 watts per machine, and 9 amps). So using the circuit for other things is a non-starter... They're dedicated to this project.
AFAIK, the Seasonic 1250W is built to handle various voltages, but I have to confirm this.
The rigs are about 40ft from the breaker box. This is in a home, but I own it, so I am free to make alterations as I will. I fully understand the issues of needing to switch out the breakers/recpticles when I move. But let's be honest here - no one is going to need 4, 20amp breakers regardless of voltage in the same compact area... Unless their either doing this, or growing pot, ha ha.
Otherwise, thank you all for helping me out thus far. In regards to the thermal issue (I think) that was asked: We're planning on adding a very large exhaust fan in between the 2 rig shelving units that will suck up and expel the waste heat outside (I actually don't have ridge vents on this house, so I get to add them to vent the attic - killing two birds with one stone!).