Just to clear up any confusion.
A coin is dropped through the coin head. This is the external metal item that allows the coin to enter the machine.
If you were going to go from a Quarter Machine to a Dollar Machine you absolutely need to change this as the dollar is bigger and won't fit in the quarter sized hole.
Going from a Dollar to a Quarter this is a "should do" not a must do.
Next the coin falls through the comparitor. There are typically two sizes of comparitors. One is a CC-16 (small token) and CC-33 (large token).
If you were using a penny, dime, nickle, quarter or similar sized token you need the CC-16. The -33 model is for Dollar coins or the larger $5 or $25 tokens.
Next the coin falls through the ABC optics. They are named ABC because there are 3 sensors and the coin must pass by each at a constant rate of speed and will generate errors if the coin is going the wrong way.
Sometimes people would attach strings to their coins so they could pull them back up (to cheat the casino).
In order to read a coin correctly (ie a dime vs a dollar) the width and depth of the coin path must be controlled. These are simple spacers. You take a couple of screws out, add a washer if it needs to be wider or take one out if it needs to be thinner. I also think there are different places for the screws to make the coin path more narrow. Its been a while since I had to do this.
Assuming all of the checks work there is a solenoid controlled diverter that allows the coin to go to the hopper otherwise it dumps it back to the coin tray.
The solenoids commonly burn out so they are often wire tied back to allow all coins to go to the hopper.