The change appeared in a recent ISS Expedition report. No explanation yet as to if it's payload or vehicle related. It's possible a last minute primary or secondary payload was added.
Another possibility is that SpaceX needed time to complete landing legs for the Falcon 9 v.1's first stage so reusability tests can be performed.
* miscellaneous experiments, consumables and supplies for the ISS. Inside Dragon.
* Optical PAyload for Lasercomm Science (OPALS), a high-bandwidth space to ground laser communications device for ISS. In the trunk.
* High Definition Earth Viewing (HDEV), which is to assess commercial HD hardwares ability to function in the LEO environment. It will also provide live video straming over the internet. In the trunk.
Secondary payloads (deployed by F9 upper stage):
* KickSat CubeSat, which will deploy 250 KickSat Sprite picosatellites
* ALL-STAR/THEIA, a 3U cubesat from the Colorado Space Grant Consortium (CoSGC) and Lockheed Martin
* TechCube 1, a 3U CubeSat from NASA Goddard
* LMRSat (Low Mass Radio Science Transponder Satellite), a 2U CubeSat from JPL
* Hermes-2, a 1U CubeSat from the Colorado Space Grant Consortium (CoSGC)
ISS Expedition update
SpaceX-3 Prepack: Wakata commenced prepacking activities in support of the upcoming SpaceX-3 mission. An estimated 25 hours of crew time will be spent towards prepacking activities in the coming weeks. SpaceX-3 is scheduled to launch no earlier than March 1st and rendezvous with ISS on March 3rd.