If you transfer the credits to Southwest I think you will have until their expiration in May 2016 to come up with the extra points. In the meantime there's virtually certain to be a short-term sale on points, such as the recent 40% extra points and the current 35% extra points. Buy the missing points then if you see a use for the award within the following 12 months.

Some diversification of credits vs. points is useful so that you have ability to book both short-notice and long-notice award travel.