Do the two sentences differ in meaning?
I would like to know whether the change is syntactical or semantical?
The syntactic difference you've already identified as being the difference between active and passive.
The semantic difference comes from the word "could". "Could" usually answers a question regarding possibility. Without any additional context, it is hard to say if the meaning of the two sentences are the same, but the answer is that it is possible for them to mean the same thing while emphasizing either John or the car.
The first sentence would answer the question "What could John drive?" indicating the person may be interested in John's capability as a driver, or with more context could indicate John needs to drive a vehicle to go somewhere, but which vehicle is not clear.
The second sentence would answer the question "Who could drive the car?" indicating the asker is more concerned about the operation of the vehicle.
The tricky part is that either sentence could really answer either question, depending on what the speaker wants to emphasize.