I purchased a Mac 24 in October of 2021. I purchased and installed Microsoft Office 2021 suite at that time. Recently, it seems as if my Office 2021 was "upgraded" to Microsoft 365. When I attempt to open Word or Excel I get a prompt telling me I need to register my version. How can I get my Office 2021 back?
iMac 24″, macOS 13.2
All Microsoft products come with a product key you need to enter for registration. click here https://support.microsoft.com/en-us/office/where-to-enter-your-office-product-key-0a82e5ae-739e-4b92-a6f4-2ec780c185db
Office 2021 and Office 365 use the same general executables. Just one is a subscription, and the other is a one time purchase. If it's asking for registration, you will need to enter your product key as explained in the Microsoft website linked above. It does not mean it is now Office 365.
Microsoft's website has this post on issues with Office 2021 requiring activation or registration every time it opens.
Unexpected Microsoft Office 2021 conversion to Microsoft 365
