This page is meant to describe how to address the problem "missing copyright file" as meant here. The cause is that dpkg refuses to replace a directory with a symlink or vice versa (this behaviour is wanted for unrelated reasons).

Here is a possible solution, creating the symbolic link during migration in a postinst script.

The other obvious solution is not to use symlinks at all. Symbolic links created by any previous version must then be removed by a preinst script, as done here.