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.
@tomzorz_ @krollTJ Yes, that is our intent.
— Gabriel Aul (@GabeAul) December 4, 2014