This is unfortunate, but your intent to change your statement closing date to around the 25th of the month should resolve the problem for next year of including most December purchases in your points for 2014. You might even want to make it the 24th or 23rd because of the holidays. (My statement closes on the 21st of each month; depending upon the month, points have been credited between the 22nd and 24th.)

Have you looked at your complete 2013 credit card points earning from SWA? Verify that you did get a credit for Chase points in January 2013 (which would have been the bill you paid in December 2012) — you should, in fact, have 12 separate credits for your Chase card, one for each month. If you only have 11 credits, then you may have an argument as to why the amount you paid in December 2013 should count for 2013 and not 2014.