Please note that the docbook-xml port is simply a meta-port
that installs all versions of the docbook-xml Document Type
Definitions (DTDs) that MacPorts ships.

It should be possible to freely uninstall, reinstall, and
upgrade the various versions of the docbook-xml DTDs without
affecting the docbook-xml port.

When MacPorts ships new versions of the docbook-xml DTDs, this
port will be updated so that the new DTD is installed on your
system when you upgrade outdated ports.