Which of the operating systems can be upgraded to Windows 7 Professional?

Windows 7 Enterprise is already a Windows 7 edition and cannot be upgraded to Windows 7 Professional, which is a lower-tier edition.

Windows Vista Home Premium does not have a supported in-place upgrade path to Windows 7 Professional - it can upgrade to Windows 7 Home Premium or Ultimate, but not Professional.

Windows XP Professional has no supported in-place upgrade path to any edition of Windows 7 and requires a clean installation.

Microsoft's official Windows 7 upgrade paths specify that Windows Vista Business can be upgraded in-place to Windows 7 Professional because they are equivalent editions in terms of feature set and target audience. This allows the upgrade to preserve user settings, files, and installed programs without requiring a clean install.