Sorry, it doesn't work like that. If a switch is rated at 250A at 24V, it's still rated at 250A at 12V. The laws of Power don't apply, as the switch has no (negligable) resistance, and thus uses no power.
With a switch or similar with a rating like 250A at 24V, the current rating is with respect to the size of the conductors,contacts etc. the voltage rating is with respect to the insulation (air) between the contacts when they're open, and between seperate conductors.
Regards
Luke