Hi Steve Mussett,
To clarify, adding your Microsoft account through the activation site does not replace or remove your existing local accounts, it simply registers your Microsoft account as another way to sign in. Removing the device from your Microsoft account online will only unlink it from your account profile; it will not delete or disable the server itself. Your local administrator accounts will remain intact, though you’ll still need the correct password to access them.
If you want to continue using your local account, you can safely ignore the Microsoft account login option and sign in with the local administrator credentials. If you don’t know the password for the other admin account, you may need to reset it through standard Windows Server password recovery methods. In short, removing the device from your Microsoft account won’t harm the server, it just severs the association with your personal Microsoft profile.
I hope the response provided some helpful insight. If it clarified the issue for you, please consider marking it as Accept Answer.
Jason.