What will happen to Windows 8 previews October 26th?

Hey Microsoft Tribe,
Another Windows 8 question: if I download the newest preview of Win8 right now, what happens when it's officially released October 26th?
I suppose I'll have to pay the $40 fee, but how will that work? Will the system be locked somehow until I buy a product key? Or will I have to re-install the entire OS from a download link I pay to get? Or will there be an update from my version to the "final" version of the OS I pay to get, where I simply pay the 40 dollars and it updates itself?
This is just something I want to know before deciding to get a Win8 preview on my PC to make sure I won't be backing up/ re-installing Windows a dozen times until it's actually released.
Thanks everyone
PS: How does updating the preview in general work? Is it like Windows Update in 7 where I just have to use a single click to get the update, or will I need to do a complete OS re-install to get the newest build?