How do I make dpkg-reconfigure rewrite XF86Config-4?

["Benjamin J. Tilly " <ben_tilly@operamail.com>]

I have a locally modified XF86Config-4 file.  I would like to
'dpkg-reconfigure xserver-xfree86' and have it overwritten.
But it won't because the file is locally modified.

As stated in /usr/share/doc/xfree86-common/FAQ.gz, the logic
for refusing to write that file is:

  The post-installation scripts of the xserver-common, xserver-xfree86, and
  xserver-xfree86-dbg packages will not overwrite the aforementioned
  non-conffile configuration files if *any* of the following conditions is met:
  - the configuration file does not exist
  - the configuration file's checksum file does not exist
  - the configuration file's stored checksum does not match the one
    calculated when the post-installation script runs
  If *none* of the foregoing conditions are met, the configuration file is
  automatically updated, and if it has changed, its new MD5 checksum is written
  to the appropriate location in /var/lib/xfree86.

The FAQ does not spell out how to convince it to overwrite anyways.

It looks like I just need to run the right command to put the right
MD5 checksum in the right format in the right place.  Can anyone
tell me what those values of "right" are?  And as a follow-up,
should an answer to this question be put in the FAQ?

Thanks,
Ben