w.r.t. exhaustive testing, i wouldnt bother if i were you, since there's a patch coming out fairly soon which adds dx11 paths to the game, enables FSAA etc, which (i assume) isnt in the demo (doubt there would be a patch for the demo or an updated version but you never know). My suggestion - play the game see if you like it - dont get bogged down in the graphics.

As for the redownload thing, i would expect you'd have to redownload. Unless you can find an 'enter a product key to upgrade to full version' button somewhere in the demo...