The news was made public on Twitter by Gabriel Aul from the Windows 10 team as he received questions from followers about possible Windows XP upgrade paths:
@enoslayd No, you'll need to use the ISO to upgrade from XP.
— Gabriel Aul (@GabeAul) December 24, 2014
@jftimm_ @enoslayd Only clean installs are supported from XP at this point. You'd need to backup your data first and reinstall any apps.
— Gabriel Aul (@GabeAul) December 28, 2014