Comment 1 for bug 129038

The behavior of the postinst script will simple rename your existing config to /etc/lirc/lircd.dpkg-old. Your old configuration should be present there, unless you reconfigured lirc multiple times to choose different remotes.
Here is the snippet of what the postinst script does:
                # we don't restore the file unless the user is (re)configuring
                elif $CONFIG_CHANGED \
                        && [ -f "/usr/share/lirc/remotes/$RET" ];then
                        if [ -f "/etc/lirc/lircd.conf" ]; then
                                mv /etc/lirc/lircd.conf /etc/lirc/lircd.conf.dpkg-old
                        fi
                        cp /usr/share/lirc/remotes/$RET \
                                /etc/lirc/lircd.conf
                fi

Nonetheless, I do see that this will be confusing to some people who are upgrading lirc and suddenly get a pretty option to choose a remote. I'm attaching a debdiff that will show a more informative message