Changing a 12VDC -> +/-35VDC SMPS to a 120VAC -> +/-35VDC SMPS

Theoretically i think it can be done but there are some important things to take care of after rectifying and filtering the 120Vac, first of all you need a linear regulator giving 12Vcc needed for the control part of the supply, it's current can either be supplyed only by that regulator or you can make an auxiliary winding that will power the control circuit after starting of tha supply, so the linear regulator would work only at the start up, that would mean little power dissipation on it; the mosfets would see at least double the Vin-voltage level rectifyed and filtered, so in this case i think 500V mosfets would be needed, adaptation of the snubbers in the primary side would be needed, and rewinding of the power transformer, lack of regulation makes it simpler to built but still not verry easy and/or cheap.