In my experience, upgrades do not work well for various reasons, but theoretically, a 32 bit version of the Vista or Win 7 OS can upgrade to a 32 bit version of Win7 Enterprise, and likewise with 64 bit versions.  That seems to be the essential upgrade criterion.

Carol Caldwell-Edmonds,
Enterprise Technology Services: Client Services
Manager, UVM Computing Helpline and the Computer Depot Clinic
University of Vermont
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On 10/5/2010 9:16 AM, Larry Kost wrote:
[log in to unmask]" type="cite">Can someone tell me where to find which versions of Vista and Windows 7 can be upgraded to Windows 7 Enterprise without doing a clean install? I realize that a clean install is generally the best route, but I have some "unusual" circumstances which make a clean install less appealing.

I may come to a clean install in the end, but I would like to try an upgrade if it's possible.