As mentioned in the article I linked to above,
In follow-up answers, Microsoft said that users running Windows 8 Release Preview -- the newer of the two sneak peeks, shipped on May 31 -- can upgrade using the Windows.com upgrade tool and pay the $40.
However, the only bits that are retained during that upgrade are personal data files. Other elements, including the applications, Windows settings and user accounts that migrate from Windows 7, do not.
I would recommend taking a backup of all important personal data in any case, since you don't want to end up frantically searching for something later, only to find that it's disappeared into the ether. Your apps won't be migrated so you'll need to reinstall them of course, but portable apps will continue to work as usual. Your personal data/files should be saved in the Windows.old folder, which you can access after the upgrade. More details can be found on the Windows Upgrade site.
If you want my opinion of whether an upgrade is the best way in your case, I'd actually recommend a clean install instead using the cheap upgrade offer. I mean, if your installed apps are not going to be migrated, what's the point? Might as well start afresh with a clean slate. Just take a backup of all your data (remember, for browser bookmarks you'll probably need to sync online or export to a file), then install using the steps outlined below (also from the same article):
Can I do a "clean" install?
Yes, you can, but not using the Upgrade Assistant. Instead, here's what you'll want to do to format the hard drive before installing Windows 8 Pro.
Step 1: Use the Upgrade Assistant to qualify the PC and pay the $40.
Step 2: When the "Install Windows 8" message appears, select "Install by creating media" to create a bootable USB flash drive or an .iso (a disk image) file.
Step 3: Wait for the Assistant to download the Windows 8 Pro installation package and create the bootable media or .iso file. The flash drive must have 3GB or more of free space available.
Step 4: Burn the .iso file, if that's the path you chose, to a blank DVD.
Step 5: Use that USB drive or DVD to boot the PC.
Step 6: When you get to the screen in Setup that asks, "Which type of installation do you want?' choose "Custom."
Step 7: Select a disk partition for the OS in the next screen. At this point, you can create a new partition if there's enough space available, or reformat the drive.
Step 8: Continue with the installation of Windows 8 Pro.