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It's been awhile, and I'm sorry for the somewhat non-committal answer, but I'd go ahead and try it, myself... unless someone else posts information to the contrary here. Frankly, I suspect an awful lot of "Pro" upgrade disks are used this way, and I'd be surprised if it didn't work.
BTW, is your disk a Service Pack 2 disk, or have they got SP3 disks on the market yet? If not, be ready for the SP3 download... :)
I found this on a MS forum ...
[In order to upgrade from Windows XP Home Edition to Windows XP Professional, you must purchase a "retail version" of Windows XP Professional. An OEM version of Windows XP Professional can only be used if you wish to reformat your hard drive and perform a "clean install".]