So, our apprehensions came true regarding the new Windows 10 Mobile build getting delayed a bit in wake of it being newer than the earlier hinted Build 10575. Gabe Aul has just confirmed that they need a day or two to release next Mobile build that will be newer than Build 10575.
@hectorosort We are looking to flight a newer build than 10575 for Mobile, but will need another day or two.
— Gabriel Aul (@GabeAul) October 27, 2015
We can only hope that this build remains upgradable directly from older Windows 10 Mobile builds as was confirmed by Gabe in case of the Build 10575.