Microsoft's Michael Niehaus recently talked about the upgrade options for Windows 8.1 when it launches later this year. Niehaus says that when the Windows 8.1 Preview launches later this June, both Windows 8 and Windows RT users will receive a Windows Update notification which will allow the user to download the preview bits.
When the Windows 8.1 RTM is available, users will again receive a Windows Update notification for both users running Windows 8/RT or Windows (RT) 8.1, however upgrading will differ for both versions of Windows. Those running Windows RT/8 will be able to upgrade to the RTM version of Windows 8.1 without loosing any data, accounts or apps, much like the Windows 7 to Windows 8 upgrade process.
Users running the Windows 8.1 Preview will be able to upgrade, but at the loss of all their apps. Data and User Accounts will be taken along with the upgrade process, but user downloaded apps will be wiped and will need a re-install. This will obviously be a pain for many people, especially if you have lots of apps that you use on a daily bases. Again, if you stick with Windows RT/8, you will be able to upgrade to the Windows 8.1 RTM without the loss of anything.