If i upgrade to windows 7 beta,can i downgrade to vista somehow? ?

And, when the version is complete, do they give it to me cause i was beta tester, or I shoud stay on unfull beta and then again pay for windows i alreadz had??? thats just stupid, why everyone says to upgrade to beta?????? I know about installin it dualy to system be 2-windowsed, but everyone also says thats stupid cause ur files get messed up. WHAT TO DO??