When you buy the final version run Windows Easy Transfer and backup to an external HDD or network location. If you're using the former make sure you deselect and exclude it's drive letter during the selection process.
Once it's done boot off your new W7 CD, click more options in setup instead of upgrade, reformat the bootcamp partition and install. Once you've loaded the bootcamp drivers and updates run Easy Transfer again restoring your files and settings - voila.