Subdomain to maindomain

Hello, I have a question if this scenario is possible: I have a Maindomain, for example maindomain.com and one subdomain - subdomain.com I want to make the subdomain.com as a primary domain. I have seen functions like 'modify account' where I can change the main domain name. I have done so, First, I have removed subdomain.com from domains and then renamed a main domain to the name which subdomain had. however I see that subdomain.com is in dead domain list now and all files are saved in file manager for it. How to make so that subdomain becomes a primary domain without creating a new cpanel acc for subdomain? should I just copy all files from subdomain.com folder to public_html folder? Thank you in advance.