Hi, I've finally upgrade my windows 7 to windows 8 release preview version yesterday. However, the installation process is a bit unusual.
FYI, my laptop has two partitions:
Partition 0: Reserved for system restore
Partition 1: OS

1st Attempt:
I was running Windows 7 and boot from DVD for window 8 installation. During the first few steps, I attempt to format my partition(OS partition), but it won't let me do it.(The button grey-out/seems disabled) So I proceed with the installation and it stated that any previous windows will be store in the "windows.old" folder.

After the installation, it nows become dual-boot (I can choose between windows 7 and windows 8). However, what I really want to do is windows 8 only.

2nd attempt:
Continue from the above(Now, running windows 8), instead of booting from DVD, I now attempt to run the dvd directly. I select "don't want to keep data" for the installation.

After the installaion, it seems there's no more dual boot. Only windows 8, but the "windows.old" folder is still there, that means the partition is still not formatted!

3rd attempt:
Continue from the above, I boot from DVD(Same as 1st attempt except that I'm running Windows 8). NOW, I don't know why, but now the format button is clickable. I formatted the disk and install windows 8 -> Success

To summarize, here is what I did:
1. Windows 7 ->(Cannot format) Dual boot Windows 7 and windows 8
2. Dual boot Windows 7 and windows 8 -> Windows 8 only
3. Windows 8 only -> (Finally I can format my partition) Windows 8 only.

As you can see, I need 3 installations to install a clean version of windows 8. It seems that we are not able to upgrade directly from Windows 7 in one shot. Why?

I hope I won't encounter this problem again when I upgrade to the final version.