Don't worry about retrofitting the voltage of the light, just use relays to switch the 120VAC to each light as necessary.
The problem with that approach is that you have to consider the safety of the relay, and the high voltage wiring to it, and anything else that is mounted on the same PCB as the relay. Any project that involves high voltages and electronics in the same enclosure is potentially dangerous. Unless you're aware of all the issues and know how to address them, I wouldn't recommend that for a DIY solution. The safe approaches are to either use something like a power tail switch, where the high voltages are safely isolated from the user-supplied electronics, or convert the voltages down to safe levels. It's easy enough to get 12V lamps in any power rating you want.