Currently network configuration fails if interfaces are already
configured in /etc/network/interfaces, that is debconf will complain
that there are leass than two network interfaces in unconfigured state
and only offer to skip this step.  As this usually happens right after
installation of the package this is very inconvenient.

If there is no configuration in /etc/network/interfaces but any
interface is configured and up debconf will proceed, generate
configuration in /etc/network/interfaces.d but fail to restart
networking.service and delete the new configuration again (see
attachment).

IMHO a reasonable solution would be to present a warning at the
beginning, then bring down/flush the interfaces which are to be
configured, and remove their configuration stanzas from
/etc/network/interfaces.