Big Dave hasn't mentioned a coil up until now. You change the ignition coil from the oil filled single pole wound coil (like you used to wind wire around a nail to make an electromagnet when you where a kid) to an open air E and I coil only if you change from an inductive (points or HEI) to a capacitive discharge system (MSD, Crane Fireball, or Malory ignition box). The actual voltage on a coil is not determined by the number of windings as would be the case with an AC transformer, but by the resistance in the secondary circuit (plug wires and spark plugs). You could have a 70,000 volt, big honking yellow Accel Super Coil, bolted on your firewall (as many a ten from the late sixties did); but if your spark plugs and resister wires break down and fire at 35,000 volts that is the upper limit of the system. Having windings that could potentially yield 70,000 volts is a waste of money; and was only a marketing ploy used by Accel, since there wasn't a car built back then that could ever achieve that voltage.
The voltage is generally determined by the spark gap. But if you have spark plug wires that leak voltage then the spark will find another ground path besides the spark plug. Just raise the hood on a dark road way (at night, not under a street light) to see if your car is putting on a light show for you. If you have bad plug wires you will see the voltage arcing to the motor where ever the wires are closest looking like a miniature lightning storm.
You change the coil to match the inductance values need for your ignition box because if you do not match the coil to the box it will reduce the life of the box burning it up very quickly in some cases. MSD offers two or three different coil choices for every box they sell. If you don't buy one of their recommended coils it voids their warranty. So be sure to research the required coil and list it on your warranty claim if one of their boxes goes bad. (excessive heat and vibration also will damage the box and promote poor performance. Not disconnecting power during the install will almost surely blow it up before you ever get to turn the key).