Upgrade mechanism between Win8, Win8.1 preview and Win8 RTM

        Microsoft will release the Windows 8.1 public preview version at the Build2013 Developer Conference on June 26. Many users are concerned about the upgrade policy of Microsoft Win8.1. According to ZDNet, at the TechEd North America conference, Michael NIEhaus, senior marketing manager at Microsoft, talked about this in the "Windows RT in the Enterprise" keynote.
When the Windows 8.1 preview is released, Windows 8/RT users will receive an upgrade reminder for Windows Update. This update will be available through the Windows Store app store with a detailed update description, and users are free to choose whether or not to install the upgrade.
When the final release of Windows 8.1 RTM is released, Win8.1 Preview users will also receive similar Windows Update and Windows Store update notification reminders. Data and account information will be preserved during the upgrade, but all applications will need to be reinstalled.
After installing Win8.1 Preview, even if you roll back the system to Windows8; if you want to upgrade to Win8.1 RTM, you still need to reinstall the application.

▲ "Explosion: Win8.1 Preview Link in Win8 App Store"
As for which applications need to be reinstalled, it depends on whether the tester is using Windows on ARM or x86. For Windows RT devices, the Windows Store/Metro-Style application needs to be reinstalled; for x86-based Windows 8 systems, users need to reinstall Windows Store/Metro-Style and desktop applications.
Skip the Win8.1 preview and upgrade the system directly from Windows 8/RT to Windows 8.1 RTM without reinstalling the app. So settings, data and applications can be migrated. Microsoft officially said that users can choose to upgrade Windows 8/RT to Windows 8.1.
Michael NIEhaus said that Windows 8 to 8.1 upgrade mechanism is better than upgrading from Windows 8 beta to RTM. A Microsoft spokesperson said that the Windows 8 to 8.1 upgrade process is similar to the upgrade from Windows 7 to Windows 8 in terms of default settings, data, and application processing.
Michael NIEhaus also revealed that Windows 8.1 will reduce space usage compared to Win8. Microsoft has done work to remove old components, temporary folders, and improve NTFS compression to free up more space for user machines. 4GB of free space is required before installing Windows 8.1 Preview. After installing Windows 8.1, Win8's recovery partition will not be replaced.