The public preview of Windows 8.1 (previously codenamed "blue") will be available June 26th from the Windows store. However, if you do install the preview and then later wish to upgrade to the final Windows 8.1 release, you will need to reinstall some or all of your apps, depending whether you have Window RT or Windows 8 Pro. If you wait until the final version without doing the preview, you will not need to reinstall anything, it should be a straightforward traditional upgrade.
Windows 8 users who do not install the preview build and opt instead to go straight from Windows 8/Windows RT to Windows 8.1 will not have to reinstall their apps. All settings, data and apps will carry over, a spokesperson said when I asked. Users will be able to decide when and if they want to move from Windows 8 and Windows RT to the 8.1 versions, officials stressed.