Robert:
The wiring you have should be good. The 24 volt system will draw less amps as long as you keep the batteries charged, and in good shape. Just remember, higher voltage less amps, and amps are what you have to worry about. I do believe that the breakers are set for 40 amps. While this was good for your 12 volt motor, it may not be enough for your 24 volt system. You may have to go to a larger breaker.Not really a big deal. You can get the breaker at any good boat dealership.
Mike