Hi @Dan Bell ,
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to "Accept" the answer.
Issue:
When you joined a Windows 10 machine to Azure AD and changed the computer name before disconnecting from Azure AD, you (and other users) were not be able to disconnect from Azure AD.
Solution:
You needed to change the machine name back to the original name, reboot, and then disconnect from Azure AD. Then you were able to join the domain normally.
One additional note is that newer versions of Windows have the administrator account disabled by default, so you need to enable it and set a password or create another local admin user.
If you cannot find the original machine name you can use regedit and search for it with "Desktop-" in Computer\HKEY__LOCAL__MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Group Policy\DataStore
Thank you again for your time and diligence in reporting and sharing the solution on the Q&A platform.
If you have any other questions or are running into more Azure AD or domain join issues, please let me know. If you would also like to share details about the issue you faced when trying to add an answer to the other post, feel free to tell me so that I can pass the information along to the platform team.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.