I bought my Apple Retail Snow Leopard Family Pack today and began upgrading my computers (Three laptops and two desktops). Everything went fine and dandy with the first upgrade from Leopard to Snow Leopard on the first laptop. No problems whatsoever.

One thing that I was worried about is three out of the 5 computers already had the Snow Leopard Developer Preview installed on them (the preview from WWDC 2009). I was not sure if this would present an issue when upgrading directly to the full retail version. So I gave it a shot, and the fact is that I can verify without any hesitation, you CAN UPGRADE without any problems whatsoever. Just insert the disk and follow the onscreen prompts.

I am assuming that this would be the same situation for Snow Leopard Server if you have that as well.

Enjoy!