I know that Model As are positive ground but have heard that they can be converted to negative ground.
How is this done and what are the advantages and disadvantages of doing this?
After reading through the replies on your post, I for one am not so sure your question was answered. Maybe I can help cornfuze you even more.
Ok, first, remember that the Model A ignition switch does NOT disconnect any of the electrical flow except the Ground to the points! Modern day cars, when you turn off the ignition, you remove the power to the entire car except the 4way flashers(basically).
Now, knowing this, you can still turn ON the head lights, tail lights, and anything else that does not have an interrupted circuit from the battery on the Model A.
To change from Positive to Negative ground on a Model A it will entail moving the shielded wire from the distributor to the Negative terminal of the Coil, and, using "non-popout" ignition switch, interrupt the Positive juice from the battery to the Positive terminal of the Coil through the ignition switch.
You will also need to polarize the generator, but this is easily done by quickly shorting a wire across the input and output terminals of the Cut-out.
All the incandescent light bulbs on the car don't care which direction they get the juice from. Now IF you care to build your own LED bulbs or lighting, that DOES care which direction the juice comes from.
And before you ask, YES LEDs can be used on a 6v system, BUT the industry hasn't found enough market to make them. Myself, I have changed to 12v Negative ground and built my own LED lighting for my early Drum tail lights because everyone makes LED for the Teacup light, but no one makes LEDs for the drum!
That is the short version