You have 3 options.
8.1 -> wait to the 29th and take the free upgrade to 10 Final = Fully licensed 10 using your 8.1 license.
8.1 -> update to 10 Preview using Windows Update now -> get 10 Final on the 29th = Fully licensed 10 using your 8.1 license.
Clean install 10 from an ISO -> get 10 Final on the 29th = Fully licensed Windows 10 as a Preview license.
If you stay in the Preview program you can continue running Preview builds under that license. If however you opt out of the Preview program you will stop getting new builds, and the Preview build you are on will eventually expire (about 6 months after its release) and your PC will then stop working until you apply a valid 10 license.
Can we sticky this from Skeeter please? (or stick it in the opening post?)

