Based on your description, it sounds like Windows may now be detecting a different license entitlement than the one you previously used on your desktop. This can sometimes happen after significant hardware changes, such as a motherboard replacement, or when multiple devices with different license types are associated with the same Microsoft account.
A few things that may help clarify the situation:
- Open Settings > System > Activation on both devices and note the reported activation status and edition (Home or Pro).
- Run the Activation Troubleshooter and check whether your previous desktop appears in the list of devices associated with your Microsoft account.
- If the desktop was previously activated with a retail or transferable digital license, look for the option "I changed hardware on this device recently" during troubleshooting.
- If you still have any records of the original Windows Pro purchase, upgrade, or activation history, keep those available as they may be useful when discussing the case with Microsoft Support.
Since your desktop motherboard was replaced after many years of use, the activation service may be treating the rebuilt PC as a different device, making it important to verify which license is currently associated with each machine. If the activation troubleshooter does not offer a way to restore the previous Pro entitlement, contacting Microsoft Support with the activation details from both devices would be the next recommended step.
Please note that this is a user-to-user support forum. Moderators, contributors including external Microsoft employees cannot directly intervene in Microsoft product features or access back-end systems. Our role is limited to providing technical guidance on reported issues, requests, or ideas.
I know this has been a frustrating experience, especially after dealing with a hardware failure and rebuilding your system. Hopefully, some of the activation details will help identify where the license association changed.