Generator ends are designed to be either reconnectable or non-reconnectable.  If a generator is listed as reconnectable the voltage can be changed, consequently if it is non-reconnectable the voltage is not changeable.  12-lead reconnectable generator ends can be changed between three and single phase voltages; however, keep in mind that a voltage change from three phase to single phase will decrease the power out put of the machine.  10 lead reconnectable can converted to three phase voltages but not single phase.  For more information, here’s a informative article on changing voltages.