I am currently downloading Win 7 home premium from MSDN, which I intend to try out on a Vista PC that is in dire need of a clean install. However, I have also pre-ordered a copy of Win 7 home premium which will arrive in October and is intended for that PC. I am wondering if I can install the one I download from MSDN today and update it to use the new key in October without the need of a reinstall. I found this blog post which seems to suggest that it is possible to change keys in Win 7.
Another complicating factor might be that I am in Europe so the one I ordered may be an "E" Home Premium version (whatever that will actually mean), whereas the one from MSDN is the standard Home Premium version.