If your broker is Fidelity or Ameritrade, a drip election is automatically in the discount plan.
As of the previous distribution (Aug), if your Broker is Etrade or Vanguard, your drip shares are purchased in the open market on the pay date or the day after the pay date. No discount. At Schwab, it appears you have to make a phone call to get the discount.
At Fidelity or Ameritrade, you can drip or not on a quarter by quarter basis, providing you make or change your election before the record date (considering the internal time lag for the change to take effect). Once you elect in or out, that election sticks until you change it.
You can sell your drip shares at any time, but you must wait for the drip shares to be posted to your account in order to select them for the tax lot sold.