The information provided in the first post is about installing the software within Win7 not using the software from another OS and using the Win7 ISO file. That is completely different. This is strictly about what installs and works within Win7 itself.
It would be advised not to upgrade. The RC build, while great, is still far from perfect. I would suggest you dual boot to make sure that the hardware has drivers and that your software works.
I know i have answered this before but to say it again,
Win7 install is not like previous versions of Windows. It copies the files from the source to the HDD right from the start. Before it even makes it to the first reboot during the install all files needed for the install are already on the PC. So if you are using a DVD or going from a ISO image, it wont matter. At the time the system hits the reboot it wont need the media or image anymore. All files are located on the drive at this time.
So you can install via a mounted ISO and it will not affect you in any way shape or form.