Southwest will need to announce this very soon if they want it to generate revenue in 2013. I don't believe they can charge this fee on any ticket purchased before the new policy is announced.
They can announce it anytime they want so long as it only applies to tickets issued after the announcement. Given that few people buy particularly far in adance, particularly on WN, if they announce it on February 1, it will probably pick up most WN travel by 2/15.