I came here to document a potential fix for other users with the same problem. We had the same problem with "Change Primary User" grayed out in Intune. I noticed the issue was due to having local user accounts on an Azure AD-connected PC. After I remove the local account, I can change the primary user in Intune. You can delete user accounts by following these steps:
IMPORTANT: Deleting a user account deletes all associated data with the account. You should only do this if you are sure you no longer need the local user account, as this action cannot be undone.
1.) Type "Computer Management" in the Windows search bar and open the app.
2.) Go to the following menu: Computer Management (local) > System Tools > Local Users and Groups > Users
3.) Right-click the user(s) you would like to delete.
Note: Not all users listed are local user accounts. For example, Administrator, Default Account, and WDAGUtilityAccount, are built-in Windows accounts and do not need to be deleted. Only delete user accounts you are 100% positive you no longer need. This action cannot be undone.
4.) Select "Delete" and confirm.
5.) Restart the computer and check Intune.