I've not tried this update yet, but I can give you some general hints.
1) ISPConfig is relatively robust concerning operating system updates. I'am doing debian updates (apt-get update, not dist-upgrade) on my ISPConfig system regularily and never had problems with that.
What may happen is that apt replaces some config files during update, so ist wise to make a backup of /etc/. The most interesting files are your postfix main.cf file and the apache2.conf file, where the ISPConfig installer has added some lines at the end of the file. If they where overridden by the update, copy them from the old config files to the new files.