You cannot change the name of the Veeam Backup for Microsoft 365 server or change domains of the server without resetting the configuration.
If any of the machines with any of the Veeam Backup for Microsoft 365 components have been renamed (or its FQDN has been changed), or any machine has been added to a different domain, then all the components become unavailable to each other. If any of the listed has occurred on a server that acts as a backup proxy server, then such a server becomes Offline in the Veeam Backup for Microsoft 365 console. To make a server available, re-add it. For more information, see Adding Backup Proxy Servers.