How to auto reconfigure package when its dependency is upgraded?

Question

Say my package foo depends on bar:

foo/debian/control:
...
Depends: bar, ...

And the post-install script of foo:

foo/debian/postinst:
...
echo "foo specials" >> /etc/bar/config

Now the problem: When upgrading bar, the Debian package manager will prompt the user that file /etc/bar/config has been modified, and ask for overwrite/keep/show diff. Though, most users don't know how to choice, and will often choose the default keep old. However:

• Overwrite/Install the new version: The foo specials will be lost.
• Keep old version: The foo specials will be maintained, but package bar won't be fully upgraded.
• Show diff: After seen the diff output, you still need to choose to overwrite or keep.

My idea is,

1. Is it possible to auto trigger dpkg-reconfigure foo, when its dependency bar is upgraded?
2. And then, is it possible to set the default "merge policy" of bar to overwrite instead of keep old?