Is that zone 8a over in our corner of Shelby County?
Overall, zones have shifted north about a half-step, which the USDA attributes to "using new average minimum temperatures." They say it doesn't have anything to do with climate change. Hmmm.
Here's the U. S. map:
You can read more about it at the USDA website. They've even set up a nifty interactive map, so you can input a zip code and get the hardiness zone.