Yes, you can. I just did.
An older computer with XP Home OEM on it (long story – the vendor charged for XP Pro, claimed it was XP Pro, and the buyer didn’t know any better and didn’t find out until a coiuple of years later, when he tried to use remote support…) came in to be upgraded to Pro. Not knowing any better, I bought him an XP Pro OEM to upgrade the computer. After going through a mild crisis over the designation on the package indicating it was for Mideast/Africa, I found that it wouldn’t upgrade, it insisted on formatting the HDD and starting over. As the owner wanted to keep his programs and data, I cast about to see if there is a way to accomplish this.
Some folks say it can’t be done with the OEM version, only with the retail version. Sombunall of these thereupon offer to sell you the retail version.
A bit of digging will lead you – as it did me – to the discovery that the (apparently only) difference among the installation mechanisms of the various versions (OEM, Retail, Volume license…) results from the data contained in the file SETUPP.INI, found in the /i386 directory (I think… I’m writing this from memory, but trust me, the file is on the CD somewhere). By changing the PID data in that file, you can tell the installer (a) that it’s OEM, Retail etc and (b) whether the license key is OEM, Retail, etc. So you can make it a Retail Install with an OEM key – which is what I did, and it worked just fine.
Actually, to tell the truth, the first CD I burned told it it was Retail install with a Retail key… it offered to upgrade, but refused the key… after a bit of head-scratching and a duhhhh moment or two, I edited SETUPP.INI, made it Retail install, OEM key, and all was well!
In sum, what you do is to copy the entire CD to a directory (name is immaterial, I called mine xpprocd), then edit the SETUPP.INI file, then burn that to a new CD… and you’re in business. Install the CD as an upgrade rather than a new installation, and a half hour or so later, you’re in exactly the same shape as if you had saved all the data, formatted the drive, installed “new”, reinstalled all the 3rd party software, restored all the data… except that, of course, it’s taken a lot less time and trouble.
Here’s what seemed to me to be the best of the explanations I dug out: http://www.thetechguide.com/forum/lofiversion/index.php/t14137.html
Hope this helps someone, somewhere, sometime!