Neowin spotted a conversation on Twitter that reveals Microsoft's intent to make it possible to upgrade from Windows 10 Preview to the final version of Windows 10. As of right now, over one million users are testing the Windows 10 Preview.
Previous Windows Preview versions were never upgradeable to the RTM version and required a complete reinstall.
Clearly, then, Microsoft is planning to offer a direct upgrade path for those using preview builds to the RTM version that will ship when the OS finally launches next year. But it's important to consider Aul's wording carefully, as he does not state outright that this will be the case, but merely that it is Microsoft’s “intent” to make that happen.