Always?? as in 100% of the time? Have you actually done a scientific survey of this to back up your claim?
Usually making such a small upgrade (from XP Home to Pro) I don't see where there's gonna be great amount of hassle or potential "issues". I would agree in reformatting or doing a clean install if it was upgrading from a Win9.x os to an NT based OS... but this is not the case with mlr.
Mlr, you can simply pop in the upgrade CD when you're booted up in XP... when it prompts you whether to upgrade, click 'Yes'.