Terra MODIS Collection 6 Level 1B products will be reprocessed to correct for the long term drift observed in the reflectance trending of band 5. More details regarding the reprocessing schedule can be found here: http://ladsweb.nascom.nasa.gov/news/news080414.html

Also, information regarding this calibration change can be found here.
https://mcst.gsfc.nasa.gov/content/collection-6-information