Just for fun I decided to see if it was possible to take a fresh install of Snow Leopard 10.6.8 that used the ProBook Installer v6.1 (just installed a couple of days ago), and see if it could be upgraded to ML by running the "Install OS X Mountain Lion.app" just like you would on a real MacBookPro.
Results:
- the "Install OS X Mountain Lion.app" ran to completion with no errors, then restarted the computer (after countdown)
- when the computer restarted it was still in SL
- so, I plugged in the ML Unibeast USB (created with latest Unibeast and ML 10.8.4 download from Mac App Store)
- when the 10.8.4 Installer booted, it did not ask me any questions about language, where to install/etc, it just launched into the update process for the SL partition that I had run the update app from
- when it completed, I used the Unibeast USB to boot into the newly upgraded ML... got to the desktop OK
- then, ran the ProBook Installer v6.1 to install 10.8.4 "OS X Support Kexts" only
And... It worked.
So, in theory an upgrade from SL to ML is possible, assuming the SL install is relatively clean.
I think where users run into trouble is if their SL install is relatively old such that they used old techniques, kexts, poor DSDT, etc to get their SL up and running. In that case, they likely create conflicts between old/new and run into problems booting, etc. Without knowledge of what exactly they did with their original installation or advanced knowledge of how all this works and what kext combinations are bad, etc, most users would be unable to fix such problems.
So, I still maintain that this kind of upgrade should not be attempted unless you know what you're doing. But it does maybe give hope that a simple upgrade process (instead of clean install) might be possible going forward with future OS X updates, provided your current install uses current techniques and tools.
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