You want to remove an old Windows AD server, and it is not going to be replaced because there are e.g. only 1 or 2 clients where a common workgroup is even enough, or whatever reason there is.
You can shutdown the server without any changes, but this is not the cleanest solution, because the clients keep searching for the server. Other methods like the Windows built-in user profile copy does not always work as it should
The best solution I have seen yet is to use the tool User Profile Manager. It integrates in control settings and you can easily rename, delete, move and copy user profiles.
E.g. if you want a domain user profile to be converted to a local one, first create a new local user and then assign the existing profile to it. Then test it and afterwards you can remove the domain settings from your workstation(s).