Pokud při instalaci W8 vybereš uložení původních Windows7 do složky Windows.old, tak to jde:
Yes, it can be done if you have the original "Windows.old" directory (created during the Windows 8 setup) in your drive and a boot-able DVD/USB device for Windows 8 installation. (Already tried with Windows 7, 99% should work with Windows 8 too.)


Boot from the DVD/USB device for the 'Windows 8 Consumer Preview' installation (change your BIOS settings if necessary) [[To make a boot-able USB, first format your USB, and then copy the whole content of 'WindowsESD' directory (you would get that inside C: drive after installing windows 8 Consumer Preview) to your USB. ]]


Let the setup load


Choose languages and click 'Next'


You would see a 'Repair your computer' button; click on that


Press SHIFT+F10. This will open an elevated command prompt


Type C:, then dir to make sure you are in the right disk. You should see Windows, Windows.old, Users, ProgramData... Otherwise, try D: or X:


Move your existing Windows 8 installation by running the following commands (ignore if you receive 'The system cannot find the file specified.' and go to the next):
mkdir win8
move "Windows" "win8\Windows"
move "Program Files" "win8\Program Files"
move "Program Files (x86)" "win8\Program Files (x86)"
move "Users" "win8\Users"
attrib -h -s -r ProgramData
move "ProgramData" "win8\ProgramData"
move "PerfLogs" "win8\PerfLogs"


Next, restore your previous Windows installation:
move /y "c:\Windows.old\Windows" c:\
move /y "c:\Windows.old\Program Files" c:\
move /y "c:\Windows.old\Program Files (x86)" c:\
move /y "c:\Windows.old\Users" c:\
move /y "c:\Windows.old\ProgramData" c:\
move /y "c:\Windows.old\PerfLogs" c:\
move /y "c:\Windows.old\Documents and Settings" c:\ (*Run this command only if your previous Windows was XP)


Reboot your computer and eject the DVD / USB device

{{ N.B.: If you had installed Windows 8 Consumer Preview over WinXP SP3, you have to perform an extra step before rebooting while you are in Command Prompt console. Here it is:- First, find out your DVD/USB drive letter; lets suppose it's ( G:\ ). I also suppose, there is a directory named 'boot' (which contains 'bootsect' file) inside your DVD/USB drive. -Type 'G:' (or whatever your DVD/USB letter is) and hit enter -Type 'cd boot\' and hit enter; (you would get [G:\boot\ ] -Type 'bootsect /nt52 c:' and hit enter (for Windows 7, it is '/nt60 c:') You are done! }}

http://superuser.com/questions/395719/how-do-i-uni nstall-windows-8-consumer-preview

http://pc.poradna.net/q/view/784409-jak-odinstalov at-preview-verzi-windowsu-8-a-vratit-se-zpet-na-7