Well that's interesting, because I ended up calling to find out and the agent I got said I would have to cancel the existing ressie and then rebook using the Ding fare (which would mean taking a chance on the Ding fare still being available).
I went online and it looked like I could still get the Ding fare, but since it was only for the return trip, I was worried I'd mess up the outgoing trip somehow, and the $10 difference just wasn't worth the risk to me.
So, that leads me to another question: Would I be better next time I book on Southwest to book two one-way flight separately, so that I would have two confirmation numbers and then if I saw a lower fare just on one leg, I could rebook that and leave the other ressie alone??
I'll figure it out someday. I was so excited a few weeks ago to get the $49 internet fares that I didn't really think the whole thing through!!
Thanks!