You don't get a voucher, but you get a credit for a future flight that needs to be used within one year. (So if you booked yesterday and the price goes down, rebook and then you have until June 4th, 2013 to use the credit - which means you actually have to fly by June 4, 2013.)
I fly SW all the time and always rebook and use credits. It is very easy to do. The best times to check flights are Tues-Thurs because that is when SW usually releases sale fares.