First of all, yes you do need a new boarding pass for the trip home, but good news is to go to the front desk of your WDW Resort with your flight information, and they will be happy to print it for you. Second, Southwest has always been very easy to work with in my experience. In 2002 they played along with a huge surprise I had for my three kids. We had told a little white lie to them, and said we were flying to Pittsburgh, why Pitt, I don't know, but anyhow when we checked our baggage I told the attendant there, and she called to the boarding ramp, and they took down the signs showing where that flight was going, and then put us on the plane before everyone else. Even the pilot was careful to say where we were going in his inflight announcements. They really didn't know where we were until we pulled in to the Poly, and told them.

Then it looked a little like this

Then I got

Below in my sig is a pic of my DS 12 when we were getting off in Orlando. Not sure if they will still do that with all the security now, but they have always been awesome. I even don't mind being on hold when I call (usually for group sales). It does amaze me that it takes as long to fly from Nashville, as it does from Providence, RI.