As I see it, you have four options here (and I'm assuming that you have no low cost carrier options in your market - they do one ways). First, buy the tickets as two one ways with miles - it's far cheaper to change mileage tickets. Second, buy fully flexible tickets - wildly expensive. Third, investigate each airline's change fees and go with the best option. Fourth, buy the return for the later date and, if you want to come back earlier, simply buy a return for the earlier date from MSY-LED and either nest the tickets, or ditch both return segments.

As to finding out about the change fees, I think Expert Flyer might help - I'm not a subscriber so can't be sure. But otherwise it's simply a question of looking at each airline's rules for each type of ticket. Broadly, the more you pay, the lower the change fees will be.