How to connect dc generator to electricity power through a change over switch

On what? Commercial power is AC, and you would never hook a DC generator to it... what would you be using that takes DC that you would have a change over switch on?

Most places that have DC needs would have a battery stack feeding it with inverter/chargers keeping them charged correctly... and generator that would be needed for long commercial power outaged would typically be an AC generator and it would supply those inverter/chargers. Otherwise there would be a power interuption during the switchover.