Since the solenoid isn't engaged and drawing excess current (due to twice the voltage applied) for a very long time, it "could" maybe might lol last a long time. However, it would be easy to add a series voltage dropping power rated resistor in series to reduce the 12 volts down to 6 orrrrrrrrrr fairly cheap n easy to just get a 12 volt solenoid if and when an old 6 volt burned out.
If changing polarity the coil polarity also needs to be changed as well as any ammeter leads so it dont read bass ackwards.
You can EITHER add a series voltage dropping (12 down to 6) Ballast Resistor to the 6 volt coil OR get a new full true 12 volt coil no ballast required
The old 6 volt starter will work and still turn same direction regardless of polarity, however, it can be a bit tougher on starter drives/gears etc when using 12 volts on a 6 volt starter. Ive done that with no problems however and dont let them crank on and on and on to risk overheating.