To transfer a site from the main domain to a subdomain within the same hosting account:
If the target subdomain has not yet been added in the "My sites
Via file manager
clear the contents catalog
subdomain and move all the contents of the main domain directory into it (or simply rename the subdomain and main domain directories by swapping them).
If the site is bound to a domain address, replace the main domain address with the subdomain address and correct the paths in the site configuration files and / or its database (instructions: WordPress
Since the transfer is performed within the framework of one hosting account, you do not need to transfer the database and change the connection settings to it.