I actually haven't been charged by Southwest for making changes. I've done it twice and I do believe it was for the same rate. I have been with other airlines that do charge a ridiculous amount. Another reason why I am a SW fan!
I just changed my return flight for July. It was very easy. I just cancelled the original flights, and the credit was immediately available. I was able to rebook for the new date at the same price as the one I had cancelled.