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. In case you did replace a symlink with a directory in version 1, but are adding the preinst script a posteriori in version 3, a system with version 2 may still contain a symlink (because dpkg did ignore your changes in a normal upgrade path) or not (because the user did try version 3 then downgrade). You should then remove any existing symlink, whatever the upgraded version, as done here.