Is it true that I can change a flight to up to one year after original purchase?
I purchased 3 tix on May 2 2014. Now I want to change ONE to 2015.
a) Can I change just one passenger’s flight?
b) We would have to complete the r/t travel before or on May 2 2015, correct?
Appreciate the help, paging @Mr_Itty though as he’s always on top of SW questions. 
You have the dates correct. We’ve changed just one passengers ticket in the past but we had to separate them from the rest of the reservation first. I don’t remember how we did that though. Sorry.
If you booked them as one booking, with both people sharing one confirmation number, you may have to call them. I’m not 100% sure, but I don’t think you can change just one person of a shared confirmation online. You can try it and see what happens, however - go to the website, “Change Flight”, and enter the confirmation number and the passenger name. When it pulls up your reservation, see if there are checkboxes next to the passengers’ names that allow you to select just one. If not, you will have to call Southwest at 1-800-I-FLY-SWA to make the change.
When you change the flight, if the new flight is more expensive than the original booking, you’ll have to pay the difference. If it’s less expensive, you’ll get the difference back in Ticketless Travel Funds, tied to the original confirmation number, which will have to be used (ie, flights completely flown) by May 2, 2015.