replacing multisite subdomain with new toplevel domain

I have set up multisite on wpengine and I want to replace one of the subdomains ( to a top level domain ( I asked the hosters of to update their a-record to When I go to add the mapped domain I get the "domain name is unavailable to access" error. What am I doing incorrectly? Do I need to ask them to add a CNAME as well and if so, to what?