Yes, administrators on XenForo can manually change a user's name. XenForo includes a built-in feature that allows administrators to edit a user's profile, including their username, email address, and other account details. This is typically done through the admin control panel (ACP) and may require appropriate permissions.
If you're asking whether moderators (often referred to as "jannies") can do this, it depends on how the permissions are set. By default, only administrators have access to the ACP where username changes can be made. However, administrators can grant specific permissions to moderators that might allow them to perform such actions, depending on the forum's configuration.
If you're concerned about this happening without your consent, you could check the site's rules or policies or contact the site administration for clarification.