upgrade/migration from a directory to a link broken

Binary package hint: dpkg

Upgrade (or migration) from a directory to a link is resulting in an empty directory instead of a symlink.

I have the following files in my install tree :

====
debian/tmp/usr/lib/foo/bar1/file1
debian/tmp/usr/lib/foo/bar1/file2

debian/tmp/usr/lib/foo/bar2/file3
....
====

I also have the following debian/foo.install dh_helper file:

=====
debian/tmp/usr/lib/foo/bar1
debian/tmp/usr/lib/foo/bar2
=====

I build and install the deb, all is good.

Now I decide to move /usr/lib/foo/bar1 to /usr/share/foo/bar1 and keep only a link in /usr/lib/foo (so that the application continues to work).

I change debian/foo.install to:

====
debian/tmp/usr/lib/foo/bar1 usr/share/foo
debian/tmp/usr/lib/foo/bar2
====

and I add debian/foo.links with:

====
usr/share/foo/bar1 usr/lib/foo/bar1
====

I build, look at the deb using dpkg -c foo*.deb, it looks fine (files and link).
Now I install that deb.

I get /usr/share/foo/bar1 correct but /usr/lib/foo/bar1 is an empty directory instead of the expected symlink that I can see in the deb.
/usr/lib/foo/bar2 is correct, as is everything else.

================

I've got that situation many times before (firefox, xulrunner, seamonkey, prism, kazehakase, ..), here is xulrunner-1.9
when I've moved the plugins dir from /usr/lib/xulrunner-1.9b1/plugins to /usr/lib/xulrunner-addons/plugins:

ix:~/bzr/build-area$ dpkg -c ./xulrunner-1.9_1.9~b1+nobinonly-0ubuntu1_i386.deb | grep plugin
drwxr-xr-x root/root 0 2007-11-20 23:49 ./usr/lib/xulrunner-addons/plugins/
-rw-r--r-- root/root 8748 2007-11-20 23:49 ./usr/lib/xulrunner-addons/plugins/libunixprintplugin.so
-rw-r--r-- root/root 18376 2007-11-20 23:49 ./usr/lib/xulrunner-addons/plugins/libnullplugin.so
lrwxrwxrwx root/root 0 2007-11-20 23:49 ./usr/lib/xulrunner-1.9b1/plugins -> ../xulrunner-addons/plugins

ix:~/bzr/build-area$ sudo dpkg -i ./xulrunner-1.9_1.9~b1+nobinonly-0ubuntu1_i386.deb
(Reading database ... 38489 files and directories currently installed.)
Preparing to replace xulrunner-1.9 1.9~b1+nobinonly-0ubuntu1 (using xulrunner-1.9_1.9~b1+nobinonly-0ubuntu1_i386.deb) ...
Unpacking replacement xulrunner-1.9 ...
Setting up xulrunner-1.9 (1.9~b1+nobinonly-0ubuntu1) ...

ix:~/bzr/build-area$ ls -l /usr/lib/xulrunner-addons/plugins/
total 32
-rw-r--r-- 1 root root 18376 Nov 20 23:49 libnullplugin.so
-rw-r--r-- 1 root root 8748 Nov 20 23:49 libunixprintplugin.so

ix:~/bzr/build-area$ ls -l /usr/lib/xulrunner-1.9b1/plugins
total 0

ix:~/bzr/build-area$ ls -ld /usr/lib/xulrunner-1.9b1/plugins
drwxr-xr-x 2 root root 4096 Nov 20 23:54 plugins/

To solve that, I have to either remove the package 1st or remove the empty dir afterwards and force a re-install.