Which two Windows operating systems cannot be upgraded to Windows 8.1 64-bit?

Upgrading from Windows 7 32-bit to Windows 8.1 64-bit is not possible because a direct upgrade path from a 32-bit version to a 64-bit version of Windows is not supported. An upgrade needs to occur within the same architecture. Therefore, to move from a 32-bit operating system, the user would need to perform a clean installation of Windows 8.1 64-bit instead of an upgrade.

On the other hand, systems running Windows 10, Windows 8, and Windows XP 64-bit can be upgraded to Windows 8.1 64-bit, as they align with the requirements for a valid upgrade path. Windows 10 is newer, while Windows 8 is the immediate predecessor to Windows 8.1. Windows XP 64-bit can be upgraded because it uses a compatible architecture, making it eligible for the upgrade process.