I agree that Area D will be very interesting.
Chaparral will provide some relief to Harker Heights and Ellison, which are currently way above capacity. Those two will either be really small 6A's or really big 5A's, depending on UIL's cutoffs, and depending on how many people who are in the Chaparral attendance zone stay at their current school. Killeen and Shoemaker will probably remain 6A, but will just be smaller 6A schools.
Belton and Harker Heights did pretty well at Area this year (I'm not sure about Belton but Heights got the highest scores and placement in their history, and were 1 ordinal point away from Belton) ranking 11th and 12th respectively. Seeing them going down to 5A will make 5A Area D interesting, especially with Dripping Springs and Weiss going up to 6A, and Lake Belton moving up to 5A after going to state 2 years in a row.
Correction: All of Killeen will most likely be 5A except maybe Harker Heights unless they raise the cutoff above 2285