directory permission not changed by dpkg

[Bill Allombert <allomber@math.u-bordeaux.fr>]

Hello developers,

I would like to change the permission of a directory in a new
version of my package, but it seems that when you upgrade a 
package, dpkg keep the old permissions on directories.

Specifically /var/games/toppler is 755 root.games for now. I would like
to change it to 770 root.games in the next version.

It is correct ? on purpose ? Where can I find a good documentation
on dpkg (dpkg-doc has very few information) ?

Is there a work around ? I can use postinst script, but how can I
handle downgrade ?

Thanks for your help,

-- 
Bill. <ballombe@debian.org>