No, there is no command that will rename both the file on-disk and the buffer at the same time. There are some ways to reduce the amount of typing you have to do, though. For example, Vim will expand % to the name of the current buffer/file, so you could use
:!mv % new_file
If new_file has the same root name as old_file, you could do this to change just the extension:
:!mv % %:r.xhtml
%:r expands to the name of the current buffer/file without the extension. See
Once you have the file name changed on-disk, you can change your buffer to that name with
Both commands have file name completion, so you can type just the first few letters of the file name, then <tab> to have Vim complete the name.