Microsoft will support full upgrades to Windows 8 only from the three-year old Windows 7, according to a report Thursday.

Yesterday, ZDNet blogger Mary Jo Foley, citing unnamed sources, said that Microsoft has informed select partners of the upgrade paths to Windows 8.

Microsoft has not yet set a release date for Windows 8, but most analysts expect it to go on sale this fall, most likely in October.

The upgrade paths that Foley's sources spelled out were the same that Microsoft revealed in February when it released Windows 8 Consumer Preview, the first public beta.

Microsoft said then that only Windows 7 PCs are eligible for a full upgrade to Windows 8, one that retains applications, data files, user accounts and Windows settings.

Windows Vista and Windows XP machines can be upgraded to Windows 8 -- assuming the hardware meets the system requirements of the new OS -- but cannot bring along all the bits. Vista users who upgrade will retain user accounts and files, as well as Windows settings, but not already-installed applications. XP-to-Windows 8 upgrades preserve the least amount in a move: User accounts and files only.

Microsoft listed the same upgrade paths in May for the later Windows 8 Release Preview, describing what was kept during migrations to the final public sneak peek in an FAQ.

Earlier, however, the company had declined to comment on whether Vista and Windows XP PCs could be upgraded to Windows 8 after it had disclosed the latter's editions that it will offer customers.

Foley's sources did say, however, that some migrations would be barred or would retain little of the PC's contents.

Cross-language upgrades -- from German to English, for example -- will only retain data files, while cross-architecture moves -- from 32-bit Windows to 64-bit -- will be offered no upgrade option, but must instead do a "clean" install that wipes the contents of the hard drive.

While Microsoft may be revealing upgrade paths to some partners, it has been much more reticent to keep customers informed than three years ago when it rolled out Windows 7.

Among the details the company has not disclosed are the on-sale date and the pricing of the two retail editions. By this time in 2009, Microsoft had revealed both: On June 2 that year, it pegged a launch date for Windows 7, and by June 25 had not only posted prices for the operating system but had also kicked off a pre-sale that discounted upgrades by as much as 58%.

Microsoft has said that things would be different with Windows 8, telling customers that it would not necessarily hew to the Windows 7 schedule for this year's OS.

The increased secrecy from the company, which historically has been much more open with information than its operating system rival, Apple, was demonstrated best last week, when it unveiled its first-ever tablet, the Surface, but left many questions unanswered, including the price, sales date, and even the hardware's battery life.

According to other Web reports, Microsoft will announce Windows 8 RTM, or "release to manufacturing," next month, most likely on July 18.

RTM is the final development milestone and means that the code is completed and ready to ship to computer makers to install on new PCs.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is gkeizer@computerworld.com.

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