Large Office 365 tenants with more than 10,000 seats can now use the SharePoint Online site swap feature to replace an old root site with a new communications site. The site swap must be done with PowerShell and needs a new version of the Invoke-SPOSiteSwap cmdlet. Once you prepare your new site for swapping, everything goes smoothly.
https://office365itpros.com/2020/03/05/sharepoint-online-site-swap-large-tenants/
