To get the lower fare, you just have to change your current reservation. the exception would be if a DING fare comes out which then you have to cancel your existing reservation and re-book.
The best way to do this is to go through the steps to make a new reservation all the way until the payment is required. then, in a new window or by phone, cancel your current one. That way you can use the funds to pay for the lower fare. Hope this makes sense. I do it all the time as we frequently travel on SW.
There is no limit on the number of times you can lower your fare. However, do be aware that the credit expires one year from the original date of purchase, not the day you get the lower fare. After the one year, they will usually give an extension in the form of a travel voucher for 6 more months but that is a little more difficult to use.