I have created a site that is mycompany.force.com that hosts a rather old public knowledge base. Now I want to make a community and the first step is to choose a domain. The problem is that mycompany.force.com is already being used as a site. Is there a way to move the domain to the community or have the community use that domain instead?
You will need to contact the Salesforce support if you want to use the same domain name for your community, that you already have your site on.
From the documentation:
Can I use the same domain name for my Salesforce Sites and my Communities?
No, you can’t use the same domain name.
Salesforce Sites and Communities must each use a unique domain name. If you’re using a domain name for your Salesforce site and you want to use it for your communities instead, contact Salesforce Support for assistance with renaming the domain.