It's not immediate apparent what the difference between the two is, but according to company responses on Facebook, they're "even better than before."
Visually, Popcorn Nuggets seem to be a little bigger than Popcorn Chicken but not as big as the previously offered Bites. They're made with cuts of white meat chicken rather than formed nuggets with an Extra-Crispy breading.
You can find them in in a $5 Fill-Up meal along with mashed potatoes (or other side), a cookie, a biscuit and a medium drink. They're also available in a KFC Go Cup along with Potato Wedges in a portable, to-go container. They're also meant to replace the cut-up chicken strips currently found in KFC's Famous Bowls, which suggest they'll stick around permanently (unless they sell poorly).
Advertising for the new nuggets looked to be aimed at kids (or rather, feature kids and are aimed at their parents):
Nutritional Info - KFC Popcorn Nuggets - Large (170g)
Calories - 570 (from Fat - 300)
Fat - 33g (Saturated Fat - 4g)
Sodium - 1890mg
Carbs - 37g (Sugar - 0g)
Protein - 30g