So this is not a guarantee, but from working with other beta builds that have functioned similarly to how SD has run the Fleet battles beta,
once the release Fleet Battles goes live the install will grab the beta files update them, and use the ones it needs saving you download time.
Most likely once the final game is installed the final step of the installer will be to clean up all the beta files still on your PC.
Soooo... wait until Sept 20???