They didn't say they're going to use "8.1" on the UHF allotment. You cannot assume its going to be 8.1, because they haven't announce what virtual channel they're going to use. As long as they still own WFAA on RF 8, it's still going to be VC 8.1 on WFAA. It could be 8.4 or 8.8, etc., but it's too early to tell what VC they're going to use on the UHF once the KMPX deal is completed.
Now Estrella's option agreement to get VHF 8 (and its just the VHF assets (antenna/transmitter, not WFAA's studio or programming), that transaction will come after the KMPX transaction is completed. And that option will last for five years after closing. And then once that option transaction get exercised and completed, you'll likely see a callsign and virtual channel swap.
Post-Option Closing:
WFAA - VC 8.1 - RF 30 - Decatur - Tegna - ID 73701
KMPX - VC 29.1 - RF 8 - Dallas - Estrella - ID 72054
So Tegna is paying Estrella to go on the UHF allotment, and the Option price for Estrella is $5M. So Estrella is essentially pocketing $14M to move to VHF (should they exercise on that option).
This was similar to the KNSO/KGMC facility swap that happened earlier this year when Telemundo swap sticks with Cocola, getting Telemundo off VHF 11 to UHF 27.