title: Save file with new filename: append to existing filename
body: <p>Is there a simple way to (in VIM) do save the currently open file with it's current name plus an appended phrase?  </p>
  
  <p>IE, from <code>/home/affert/</code> type <code>vim /data/folder/file1.txt</code></p>
  
  <p>then save the file as <code>/data/folder/file1.txt_BACKUP</code> without needing to copy and paste the filename?</p>
  
  <p>Context: I have a file that has full paths in it to other files in other folders.  I use <code>ctrl+W, ctrl+F</code> to open the file in a new window.  That's why I don't want to copy and paste.  BTW, the folder and file names are a lot longer, so typing them myself is not a useful option.</p>