in my experience bipolar makes more torque than unipolar.. my first question would be is the drive current the same? and my second is are you running in full step, 1/2 or what? also, you need to know if you are running full step, are you running 1 coil on at a time, or 2?
as for your existing motors.... a stepper will live forever, well almost, the bearings/bushings would be the first to fail, assuming that your not cooking them with too much current... so long as your not cooking them, and the bearings/bushings are good, then theres really nothing else that can fail.. its just a bunch of wires wound up, and some magnets attached to the shaft..... i suppose there is a remote possibility that the magnets have lost some of thier pull, but your conversion should have no effect on that one way or the other.... i would check your step mode, and how many coils your firing at a time... simplest fix to start with...
Grizzly X3, CNC Fusion Ballscrew kit, 3 500oz-in bipolar steppers, 3 203v Gecko's, Linear power supply from Hubbard CNC, Mach 3, BOBcad Pro Art V22, Rhino.