Well my "research" was seeing that there were only 2 odd evening flights from ATL to IAD, looking at flights to Reagan and choosing one that I liked, calling the agent. I figured I'd deal with rental car logistics later. Well I did a quick search and saw that prices were much higher and realized my mistake. Obviously, if I couldn't change the flight I would have just eaten the cost of the rental car but I'm glad it worked out. I did have to do a good bit of pushing. It worked out well this time but there could have been a much less ideal outcome. I don't fly Southwest too often but they're not exactly on my happy list due to this. Oh well, hopefully the flights are smooth and they can redeem themselves.