Professional to Home

i am trying to uninstall a copy of professional on my home pc, when i put in my home edition cd rom it comes up with you cannot roll back but still lets me continue with the setup, but i dont know where to install it to when it gives me options with partitioned hard drive spaces. but if i override the existing OS will it erase all my data..? i have so much data i dont want to lose and alot of files are 1gb+ so writing it all to disks will be a huge pain.