I have upgraded to every leaked build since I was first given Milestone3 on this laptop and it has generally been fine, you may well need to modify a file if the installation process says it can't upgrade from that particular version but from what I have seen it works fine.
Regardless, you should definately back up any important data before you begin although even the clean install will put your old installation into WINDOWS.OLD unless you specifically format the drive during the installation.
The leaked Windows 7 E with no IE8 shows the upgrade screen ready to roll......
"Windows 7 Ultimate cannot be upgraded to Windows 7 Ultimate E. You can choose to install a new copy of Windows 7 Ultimate E instead, but this is different from an upgrade, and does not keep your files, settings, and programs. You’ll need to reinstall any programs using the original installation discs or files. To save your files before installing Windows, back them up to an external location such as a CD, DVD, or external hard drive. To install a new copy of Windows 7 Ultimate E, click the Back button in the upper left-hand corner, and select “Custom (advanced)”.
"Windows 7 launches in the UK on the 22nd October and the main version sold within the UK will be the “E version”. This includes Windows 7 E Home Premium, Windows 7 E Professional, Windows 7 E Ultimate and Windows 7 E Starter editions. This version of Windows 7 (along with the N version*) will not include a browser (for example: it will not come with Internet Explorer 8). We recommend therefore you have an Internet browser from Microsoft or from another software manufacturer saved onto a CD/DVD or another medium before you install Windows 7."
After a year and a half with a Mac, I'm coming back to the Windows world with a Dell Studio XPS 435t. With that purchase I qualify for a free upgrade to 7 when it comes out in October. Might any of you chaps know if I will be able to go to that version from the RC? I haven't received the computer yet and I would love to start out with a full (legal) version of 7 RC as I can't get a legal version of RTM. Any ideas?