I see nothing wrong with using an upgrade copy for a clean install as long as you had a valid copy of the previous OS and can prove it. Prior to Windows Vista, all you had to do was insert your old CD or your old serial number and off you went. With Windows Vista there was a double-install workaround.
According to past reports, it appears the same workaround will work for Windows 7. Somewhat of a blessing, even if you have to install twice, rather than having to wait 20 hours. Of course, you'll have to reinstall all your applications.
According to the blog post, the exercise was to determine if a Vista SP1 -> Windows 7 upgrade was within a 5% threshold faster than an upgrade from Vista SP1 -> Vista SP1. Microsoft chose to use a Vista SP1 -> Vista SP1 upgrade instead of Windows XP -> Vista as their baseline because:
- Windows XP is a vastly different operating system compared to Vista and an upgrade from Windows XP -> Vista would not be a good comparison with Vista -> Windows 7
- Windows XP did not support 64-bit upgrades and we wanted to track 64-bit upgrade performance as well as 32-bit upgrades for Windows 7
- Vista SP1 -> Vista SP1 is a valid upgrade path that exercises all upgrade code (this upgrade is commonly used by Product Support Services for a repair scenario)
Clean
No data and 0 applications
Vista SP1 -> Vista SP1
Low End Hardware: 32-bit: 40.79 minutes; 64-bit: 57.19 minutes
Medium Range Hardware: 32-bit: 31.20 minutes; 64-bit: 39.09 minutes
High End Hardware: 32-bit: 29.35 minutes; 64-bit: 36.66 minutes
Vista SP1 -> Windows 7
Low End Hardware: 32-bit: 39.30 minutes; 64-bit: 46.51 minutes
Medium Range Hardware: 32-bit: 29.75 minutes; 64-bit: 33.03 minutes
High End Hardware: 32-bit: 26.75 minutes; 64-bit: 30.28 minutes
Medium User
70Gb of data and 20 applications
Vista SP1 -> Vista SP1
Low End Hardware: 32-bit: 179.19 minutes; 64-bit: 217.65 minutes
Medium Range Hardware: 32-bit: 117.73 minutes; 64-bit: 111.15 minutes
High End Hardware: 32-bit: 106.80 minutes; 64-bit: 101.03 minutes
Vista SP1 -> Windows 7
Low End Hardware: 32-bit: 170.41 minutes; 64-bit: 177.00 minutes
Medium Range Hardware: 32-bit: 99.65 minutes; 64-bit: 92.40 minutes
High End Hardware: 32-bit: 99.65 minutes; 64-bit: 83.56 minutes
Heavy User
125Gb of data and 40 applications
Vista SP1 -> Vista SP1
Low End Hardware: 32-bit: 361.15 minutes; 64-bit: 390.97 minutes
Medium Range Hardware: 32-bit: 194.79 minutes; 64-bit: 186.05 minutes
High End Hardware: 32-bit: 176.37 minutes; 64-bit: 172.37 minutes
Vista SP1 -> Windows 7
Low End Hardware: 32-bit: 343.36 minutes; 64-bit: 345.88 minutes
Medium Range Hardware: 32-bit: 187.30 minutes; 64-bit: 164.85 minutes
High End Hardware: 32-bit: 159.50 minutes; 64-bit: 151.25 minutes
Super User
650Gb of data and 40 applications
Vista SP1 -> Vista SP1
Low End Hardware: N/A
Medium Range Hardware: 32-bit: 1305.72 minutes; 64-bit: 635.54 minutes
High End Hardware: 32-bit: 768.24 minutes; 64-bit: 611.61 minutes
Vista SP1 -> Windows 7
Low End Hardware: N/A
Medium Range Hardware: 32-bit: 1214.86 minutes; 64-bit: 608.07 minutes
High End Hardware: 32-bit: 672.87 minutes; 64-bit: 545.93 minutes
Honestly, the statement about a Windows 7 upgrade taking 20 hours is a bit hyperbolic. I wouldn't consider the Super User profile to be realistic. Medium User, however, isn't an outlier, and comparing a clean install vs. that profile still shows a large difference in times.
Stability wise, I always recommend a clean install. Even excluding the Super User profile, this study / experiment just gives users more reason to do so.
