Choose <Yes> only if you are sure that by overwriting the
contents of the file on disk with the file contents in
memory you won't lose valuable information.
Otherwise,
  - Choose <No> to redisplay the dialog box and enter a
    new, unique disk-file name for saving the file in
    memory. Or,
  - Choose <Cancel> to stop the command, leaving everything
    just as it was before you chose this command.